Start with this to get an idea how the system works. Once you understand that try to match it up with other parts
http://upload.wikimedia.org/wikipedi...s/4/44/AEG.gif
Gears, piston, cylinder, air nozzle, tappet plate, spring, spring guide, etc.
Then watch a couple youtube videos with take downs and when you go to open up your gearbox do it in a clean area and try not to let it explode and lose random parts in your carpet.
As far as upgrades goes, try to understand how the system functions and how the parts interact with each other first. Then you can worry about upgrades and such. As for tools, pretty much if you do DIY hobbyist stuff you'll be fine. Otherwise I recommend a multi headed screwdriver set (
something like this), some hand tools (mastercraft or whatever) pin punches, angled pick, awl, etc., and have at it. Something else that's useful is an AR wrench don't buy an airsoft one buy a real steel one, they're cheaper and probably of better quality too.