Best way to learn is to have someone else show you the ropes. It's all pretty much common sense and if you have the a bit of mechanical aptitude and patience...you'll be alright.
Failing getting hands on time with someone who knows the ropes...research, research, research.
Get a spare mechbox or one of the cheap clones and dive in. Go slow, try to figure out what part does what, make small incremental changes.
I learned by researching and experimenting. I blew several mechboxes on purpose to see the effects of different components (watch plastic bushings blow on a 1300+rpm mechbox is kind of fun in a destructive sort of way...).
Work on every type of rifle you can get your hands on...it's good to learn the ins and outs of different models. M4's Ak's G36's and MP5's are the mainstream rifles.
You have to have enough time and patience to spend hours doing very fiddly work...it's not very exciting. I've found the #1 reason for a poor repair is rushing it. You cannot just slap in parts and expect the whole rifle to work well.
When you get to the point where you're very familiar with what you're doing...then you're faced with some decisions:
1. Do you really want to work on someone elses stuff? Repairing/tuning your own is fun...repairing someone elses is work. Repairing a half dozen M4's in one week...is boring.
2. Are you prepared to cover the cost of someone elses rifle if you break it? You should be...your customer is entrusting you with their pride and joy and expect your best. If you have $4000+ in other people's AEGs...can you cover that if something goes wrong?
3. Do you have the time? Opening yourself up for doing repairs will consume every spare moment that you have. There's running around getting/finding parts, picking up and dropping off, meeting with people to have them drop off their stuff, doing the actual work, testing, etc....
I am not an expert and I'll never know everything there is to know about AEGs. I know what I know and am willing to work on someone elses stuff. I'm also willing to take the time to show other people what I know...and from their feedback, I'm a decent teacher. I'll tell someone flat out that I cannot help them if I know I can't fix it. I always try to be fair and up front. I do it for the challenge and opportunity to learn new things. There isn't a whole lot of money in it...every penny I make at it seems to get put right back into other airsoft stuff (and maybe a shiny new toy every now and then).
Best of luck,