I think this is a perfect place for a native plant and hummingbird garden. First, it appears that the area gets only moderate water in the summer (otherwise I think you'd have a lot more weeds). This is good for most natives. Second, this is an area that would look good in a semi-natural state. It isn't going to be a manicured garden, but colorful flowering plants that grow well in the existing conditions are needed.

I would plan to use large groupings of relatively few types of plants. You have lots of space and could choose dozens of plant species, but fewer plant species would create large swaths of color, which is good for viewing from a distance, and larger groups of a single type of plant give a more uniform appearance.

Here is how I would start. First, starting in the area shown in the top photo, I'd clear all vegetation along the top of the wall up past the small tree (which I would probably remove - I think it is a Crepe Myrtle which I don't like in the first place, and which doesn't look too healthy). I would remove all vegetation and vines right up to the edge of the thicker vine cover in the upper right to create a neat edge to the thick vine cover. That is, I would eliminate the area of partial vine cover, leaving only nice, thick cover and creating an area with no vegetation where I would plant something new. It is not important that the area I clear has a uniform shape, and I prefer an irregular shape for plantings like this, but I think it looks a lot neater if it has a well defined edge.

Once I have the area cleared of existing plants, I would add new ones. The area is pretty large, but I would choose only perhaps two types of plants. My choice would be a native sage - Salvia clevelandii, one of the cultivars of Salvia Cleveandii like 'Pozo Blue' or Winnifred Gillman', or Salvia mellifera - for the area in the back near the small tree, and I think Island Bush Snapdragon (Galvesia speciosa) for along the top of the wall. I would plant many of each species, spaced about two feet apart to create a large group of each species. At the edge of the group I would leave a gap of perhaps four feet between my new plants and the vines cover. This area would be for access (allow me to walk) and to keep the vine trimmed somewhat neatly. I am assuming this area is watered at least a little in the summer. I think the Galvesia needs a little summer water in your area, and the salvia can stand moderate summer water.

The salvia will grow perhaps four feet tall and blooom much of the spring and summer with blue flowers. The galvesia has red flowers throughout most of summer, and arching branches that will look nice hanging over the wall. Both are native and very good hummingbird flowers, so they'll bring a lot of hummingbird activity to the hillside. The salvia, especially salvia clevelandii and its cultivars, has a great minty herby fragrance that you will smell in the year.

In between the plants you might want to spread a mulch. I'd use shredded bark or even ground up tree branches such as those created by power companies when they are clearing branches along electric lines. The power company may give you free mulch if they are triming in your neighborhood - I think they are usually looking for places to get rid of the shredded branches they produce. I would water the new plants at least once per week all summer unless you are sure the irrigation system is doing a good job keeping them watered. After this summer, you probably won't have to water at all, as long as the irrigation system provides at least a little summer water.

There are lots of other plants you could choose, but I would definitely start with clearing an area and planting large single-species groups of plants.

I also have steep slopes on 4 sides of my house. Unless you are willing to struggle with gardening on a slope, I'd keep it a simple groundcover. Any plant that needs any kind of maintenance at all, even once-yearly pruning, is a major pain on a slope. If you have bare spots, weeds magically appear.

Have a look at Myoporum 'Pacificum', which is drought tolerant, can take sun or shade, grows fast but won't take over, and is dense enough to prevent a lot of weed growth.

Here is a link that might be useful: list of CA friendly plants for slope stabilization

Since you are new to gardening I would keep it very simple like hoovb said. I have a slope and is difficult to get anything started due to watering run off. Natives are great and I put in tons of salvias for my hummers but my slope is divided by retaining walls so a little easier for me. You are smart to tackle one area at a time so you don't get overwhelmed.
You also want to see how the sun hits during all seasons.
Those do look like gazanias you id'd. They should fill in as well as the rosemary. They are contrasting colors also so that should be nice.
Agoura Hills should be zone 21.
If that is a crepe myrtle I can't say it is doing bad since they are deciduous. Their bark in winter can be pretty. The slope has been neglected so it should be fine in a year. Birds like almost any tree so nothing wrong with leaving it there.
I just put in myoporum on part of my slope and that was recommended by a nursery.
You could also look at creeping thyme.
I wouldn't put in any ice plant no matter what kind.
If you put in a lot of thick heavy stuff it just gives rodents a place to hide. They like ivy and vines.
Pick up a Western Garden Book for ideas. Another book for ideas is Pat Welsh's Southern California Gardening.

You have similar situation to me in SoCalif with a slope. After i pulled everything out from a previous owner this is what I did.

Armstrong nursery will send out a professional to your home for $60 for an hour. I only needed one hr. They will ask you some questions after seeing the spot and map you out what you need, how many, the proper names etc. They will give you about 3 15-20% off coupons to use in like the next 6 months. I more than got my $60 back when I used the coupons. Also sign up for the rewards program. It's a red card that you get pts for when you buy stuff and when you get enough pts, you get a $25 gift cert on the spot to use then or next time you come in. Over 2 yrs I've gotten the reward 3 times. I'd go this route again if I had to do it over. Very helpful and I love the result. Good luck.

Wow! Thanks for all the helpful suggestions. I really like the idea of planting some natives and attracting humming birds. I do want to keep it fairly simple since I don't want to spend a lot of time up there maintaining it.
Thanks, too for the suggestion about Armstrong. It probably would be worthwhile for me to have someone come out and take a look at it and give me a professional opinion about what would grow well under my specific conditions.
Thanks again for the tips and advice. I think I will be hanging out on this forum for a long time while I work on this backyard project. :)

> huge bare spots that are ugly and I want to try to unify it and make it look attractive.
I wonder why the pine needles aren't forming a (thin) cover.

> I think the orange flowers are gazanias??
the clumping type. They'll (continue) dying out.

> There is also some rosemary mixed in there.
do they appear to be spreading (wriggly branches rooting)?

what are the pale scattered things that look about 2-3"? If sandstone, then you could spread a thin layer of more sandstone to help the rosemary spread.

_________

For covering purposes, I would mix the species. I don't know socal, but some of these might at least spur better suggestions...

Asparagus 'Sprengeri' is pretty good for long seasons and bright shade. Has short sharp prickly hooks on stems.
http://www.desert-tropicals.com/Plants/Asparagaceae/Asparagus_densiflorus.html

Mahonias would probably settle at the top of the wall. I don't think the drier Mahonias will fill, but could be part of a mix.

Dry coastal Salvias or hybrids (S mellifera, etc). They will probably try "running from" shade.

Myoporum parvifolium. plain green cover (low shub)
Coprosma x kirkii is another plain green cover, but I'm now looking in Sunset.. where they say it's not so dry growing as what i've seen. Maybe the one (or two?) i saw dry were just lucky. They had no competition and were well mulched, so...

Erigeron karvinskianus, sort of weedy looking, but dense. they seem to live a long time for a herbaceous-looking plant.

Geranium macrorrhizum. whiter foliage, else similar form. i've never grown it. Tough reputation. (but read the sunset description!)

Malvastrum, but it will smother anything (such as slow children) in adjoining moist areas. (top of retaining wall is good barrier) Also I don't know how well it hangs on steeper slopes. It likes water but will survive with little water once it's large (which is soon, if watered). Will lose a lot of leaves if dry, showing a lot of stems (it seems to try to smother itself :-) )
Oenothera berlandieri. nice flowers. is a strong spreader (underground), but tends to die out completely. will not stay on a slope. goes to flat ground and swales. will likely go thru that "railroad tie" part of the retaining wall, so don't plant it near there.

"Dusty Miller" Senecio(s) or Centaurea gymnocarpa. Won't live very long (on flat ground, but on a slope??), but will grow fast.
Senecio wants full sun. Centaurea might grow well under high overhanging pine branches. (still wants sun) spreads/runs irregularly, good in the mix.

Lavandula x 'Goodwin Creek' is pretty tough for mostly full sun (mixes ok with similar sized neighbors).

Teucrium fruticans. "subtly" charming (bright overall color). won't cover anything densely (is a weak shrub) Mixes well with other shrubs.

try Eremophila. Where you can see the flowers.

Ballota is tough, though small. I'd pop a few in along top of wall. Socal warm season might be longer than it likes, though.

Euryops, though often sheared (!gak!), are very good naturalistic shrubs. Essentially full sun.

Acacia redolens. Caltrans tough. Not so great looking close up, imo. More like a thick haze than dense cover. Don't know how well it mixes.

There are some larger tough plants. Plumbago. Cissus (vines). Eleaegnus. Tecomaria (you might want to keep the center pushed down.)
Large fast Ceanothus (griseus) covers, will do ok in bright woodsy edge. Good in the mix.

Research Junipers. Still very tough. I see this sometimes http://images.google.com/images?q=Kabatina+blight
Not sure about that in Socal.

http://i72.photobucket.com/albums/i199/kates_mommy/BackyardHillside-1-1.jpg
What's that thin multi branched treelet?

http://i72.photobucket.com/albums/i199/kates_mommy/BackyardHillside-3.jpg
And try some "nice stuff" at the top of wall in this area...

heh. took me so long to reply, other ppl did meanwhile (actually i went somewhere for a few hours and left the browser up) :-)

if that multi treelet is crape myrtle, it's in way too much shade.

thymes will die if dry.

a few more.. helianthemum. cistus.

ccdry-
Thanks for all the suggestions.
There is a layer of pine needles- particularly at the top of the slope. We had the 12 largest pines trimmed in Dec and they raked up a whole truckload of needles.
The tree is definitely not a Crape Myrtle. (I had those at my previous home). The tree trimmer told me what it was but I can't remember the name of it.
The rosemary does appear to be spreading with wiggly rooting branches.
Sandstone? Hmmm I have no idea! I just thought it was rocky soil. There are small flat rocks all throughout the soil.

I'm off to the garden center to take a look at some of the plants that have been suggested.
Thanks!