This is my career (motorcycles and ATVs, some SxS and a smattering of other small engine things). I did MMI (Micky Mouse Institute/More Money Institute) more than 2 decades ago... learned exactly 1 thing (it's all the teach). Read the service manual. There ya go, a $30k education. Pick up a model that's common, something that's been in production a LONG time. Bonus if there's an established web forum with an FAQ (or at least some technical writeups). BUY THE OFFICIAL SERVICE MANUAL. You can get a Haynes/Chilton too, but do not skimp on the OEM. Then search to see where the mistakes are in the forum. Tear it all the way down, engine to crank, build it all the way back up. Replace every bearing and bushing. Even if you spend $10k (that'd take a LOT of work, going out of your way to find things to spend money on), it's the cheapest education you'll ever get. As a bonus, a lot of times it's the people making their own way without "formal education" that find the better ways because they haven't been taught that you "can't do that". Don't go nuts on tools, I did the lions share of my learning with less than $100 in tools. I STILL do the lions share of my work with a set of basic tools (combo wrenches, ratchet and sockets, screwdrivers and the odd pliers), only buy the specialty tools as you need them... if you can't borrow them or do the job with an alternative. It's not good money, it's crap work, abusive customers most of the time. I quit working for others within 2 years, do fix and flips now and still rely on my wife for most of our income. I really hope you don't like ATV's, because you sure a shit won't if you try to make a career out of them.