Your first step should be determining the layout of the garden.......deciding what will go where and how the individual elements will interconnect :-) Roses and vegetables need a lot of sun - locating them well away from any trees that may cast shade will be important. Do you want any lawn? It makes a good "carpet" and counterpoint to planted areas. What about outdoor entertaining areas? Is there a patio or deck or will there be? How will these relate to the garden? Do you need privacy screening and if so, in what locations. There are lots of DIY landscaping books on the market than can help you with these issues - the box stores like HD or Lowe's generally carry a good selection or check with the library. And although they have rather unfortunate titling, the "For Dummies" series of DYI books are really quite good for novices - look for 'Landscaping for Dummies' for a good basic resource. Or you could hire a design professional to give you a hand.

Preparing the soil is a bit further down the road. Rototilling is often not the best way to get a garden going. First, many weed seeds get disturbed with tilling and fall is a prime time for their germination. And many persistent perennial weeds are not destroyed just by tilling. In fact, it can spread their root parts around and exacerbate the problem. And tilling disrupts the soil ecosystem. Many gardeners find smothering the weeds with layers of damp newspaper or cardboard and mulch very effective. The paper products prevent light and oxygen from reaching the weeds, will breakdown and decompose in time and together with the mulch, provide a reasonable planting medium. Sometimes importing soil to provide additional planting opportunities and to create some height is beneficial. And if it is a new construction property, importing some decent soil is almost a requirement.

But you first order of business is planing :-)

I agree that planning is your first step. Try to get an idea of what you want. You can start slowly though. Trying to do the whole thing at once can be expensive and frustrating.

Maybe get an idea of what you want and then set the hardscape (and irrigation if you want it) in place and work on sections at a time. That way you can also find out what thrives in your microclimate versus what doesn't. You can always put in grass or keep the weeds mowed in the mean time while you get it done to keep it from being mud.

Sure, you have to start with plan, but if for now you know nice sunny spot that can be incorporated in your plan as veggie or flower garden, you can do some work now. Mow area, check if there are no big rocks or some construction debries in upper inch or two of soil by loosing soil, water area deeply, cover with thick layers of newspapers or use your moving boxes, put on top any organic material that you can find, maybe you have neighbors who don't use grass cuttings and don't use pestisides, you can get grass cuttings from them, fall leaves (better to go with lawn mower over them at first), peat moss and so on, you can add throu the winter. You can dig in kitchen veggie only scrapts under (just make sure that they are deep or wild life can dig for them, better to chip in some way veggie scraps- some people are using blender). By spring things will partly decompost, you can add some soil on top and plant what you want.
Worked for me. I have a neighbor who is using landscaping company to do mowing and cleaning. My neighbor is not using chemicals, so i get everything that gardeners remove from her garden and use to create beds in my garden. Sometimes i poor on future beds leftovers of coffees, soft drinks and so on. Sometimes get coffee grounds from Starbucks.
Don't know how big is your garden, but if it's not very big, you can cover it tightly with black plastic (after mowing and watering), you can put newspaper, brown bags or moving boxes under plastic. Should kill weeds by spring.