This is not difficult to do by yourself (I did it when landscaping in high school - even ran new lines). First, try to get the identical sprinkler (they sell them at many home centers for about $5-$15 per head, depending on the size) - check the model number, as 2 identical looking sprinklers can have different flow rates. The number should be on top. If you can't get the exact number, you'll have to do some measuring as to how much area the sprinkler covers, shape of coverage, etc. to know what replacement will most closely match the old one.

Second, carefully dig your sod out about 4-6" on all sides of the sprinkler. Then dig the dirt down to just below the connection with the line.

Third, the sprinkler should simply unscrew, but because pipes can sometimes become brittle, it is good to backhold the line (or nipple from the line) with a light hold of a channel lock pliers.

Fourth, ascertain that you don't need an adapter to change the pipe thread size. If not, simply screw the new head on. otherwise, you'll need to get a reducing fitting to adapt the thread size (also available at virtually every home center).

Fifth, test it. If it doesn't work, check the placement (to make sure the head is aiming the right direction) or adjust if necessary (to change coverage area).

Sixth, if it works, fill in the hole with dirt, then carefully place the sod back in place and water it in heavily (and continue to water every day for a few days to make sure the roots take).

As far as the valves, they should also sell those at home centers. Make sure they are compatible (voltage-wise) with your existing timer. Not a difficult repair there, either, though you may have to replace some plastic pipe-fittings in the process (this depends on what kind of pipes you have - PVC (solvent glued fittings) or PE (flex - clamped fittings).

Good luck!

You can easily tell if you can do it all. Uncover a head down to the point where you can unscrew the top ONLY. You might need two channel lock pliers to do this. The insides should come out as a complete unit, leaving the body in place. The tube should slide up and down easily. Unscrew the nozzle or tip from the top of the tube and make sure that the filter (if there is one) and the tube are clear. Pop the valve and water should shoot up from the body still in the ground. It is quite important to allow the flow to stop on its own to prevent 'stuff' getting into the body itself. If all is well, assemble the sprinkler. If not, replace parts as necessary or the whole thing. As for the valve, there are a few things - mechanical and electrical that should be checked out first. If replacement is needed, it is easiest to replace all the internal parts rather than the whole valve. I say this because I try to live within MY limitations. Let us know how the project goes.