Do you have an AEG at the moment? And, what exactly do you want to learn? If you are just looking for the "how to" kind of stuff you can learn basically everything you need to know from here: http://www.mechbox.com/ As far as the 'what parts' kind of questions it's a lot of trial and error combined with reading reviews and threads. There is a lot to be learned from Vindis DMR guide as well as this website: http://www.airsoftmechanics.com
Hope that helps.
Hope that helps.