This is my understanding, and I have a kid here ... so I might be a little misinformed, but it works.
I live in Hillsborough County, the first year of school that your child is in the district, you have to complete the form by mail (I think you can hand deliver it) and from then on it's done on the computer. There are open dates when you can apply. The first is in late December (I think) then March, then in the summer. If your child is going to go to a magnet school, this is the same opening period. From what I understand, and this is something we have to figure out this year, but once your child is placed in their school of choice you don't have to reapply each year (this is our first year in our choice school, so I have to double check)
Now, I don't know if this differs from district to district, but this is how it works in our district. You can go to the district website and type in the address you're going to have. (There are also maps available that have the different districts as well
http://www.sdhc.k12.fl.us/schools/Map2009 - Map Side.pdf is our districts) From there is gives you your home school. You also have two "choice" schools in your area, and if your child goes to one of those three schools, you can get bus service (I think!) I don't know how to find out this information, and again, this could be different in each county.
Now, in our county, as long as the school of your choice has an opening, your child can go there. For example, our home school is Y and our two choice schools are S and D. However, we have her at R. You have to have your own to and from transportation to go to a school outside of your "choice area"
Wow, I hope that makes sense. lol. It's kinda a PITA, but it works, I guess. You usually get three choices each application period, and I don't know how it would work if you were moving after the application period is over.