Its not an easy thing to learn. There is alot of information one simply cant just pick up on a week or two. Diffrent AEGs have diffrent mechboxes and diffrent set ups For starters lets keep it simple.
Say for example you wish to build an Armalite variant AEG.
Well the first question is, do you want it front wire or rear wired? After making that decision you can purchase a mechbox. Now do you have the knowledge to work with a mechbox? if yes than perhaps you can order the MB shell and parts and build one yourself with customised parts. If you are lacking the knowledge then a prebuilt complete is something you should pick up.
Next, does this MB included a motor? If yes then your fine, if no you have to consider a few things, do you want a high speed, high torque or regular motor? If your just running a stock MB then you can just get a regular motor.
After all that is said and done you can selecter your front end, flash hider, reciever (careful importation of these is a no no) pistol grip, trigger guard and other furniture, stock and buffer tube and magazines and other accessories.
This was all just an example, the best way to learn is to try it yourself, you dont have to have deep pockets to do so in most instances but you best have alot of patience. Hell personally I think the easiest method would be to buy a cheaper AEG take it apart and try and put it back together, after that order some diffrent parts to change the look.
Your question is asking alot and unfortunetly there is no school out there that can teach you, however there is mechbox.com. They have some simple videos about the dissasembly of some AEGs and their mechboxes.
http://www.mechbox.com/site/