The sheldon brown website is excellent,
just google what you want to learn about, and any results with sheldon brown in them are worth looking at (it's easier to google his stuff than try to navigate the site IMO)
www.sheldonbrown.com
Also, get yourself a half decent bicycle manual, I have the haynes one which is OK'ish, thinking of getting the parktool one though. The books are good for out at the shed/garage.
Definitely worth getting a few tools, especially the basics, I've gone for cheaper options because I don't do loads, it may have worked out cheaper if I had bought a tool kit instead of buying individually.
A good tip I got from my manual, when checking the bike, work in a zig zag from from front axle to rear, going up via the brakes, fork headset and handle bar's and all levers etc, back down to the cranks and bottom bracket and pedals and front derailleur, up to the saddle and then back down to the brakes, rear axle and derailleur etc. Following that should mean you get to check everything on your way around the bike.
There is indeed a lot to learn, but if you have a bit of patience and ask people, you should be able to manage, just take care in your work on the bike. I only started riding again last August, couldn't even adjust my brakes (still have trouble there!) but I have now fully stripped and rebuilt an old touring bike, there were a few mistakes along the way but you learn for next time.
I found there is a lot of incompatibilities with bike parts, so be sure to check details before shelling out, can be expensive when buying online and getting stung for restocking fee's.
Just ask in this forum or in the know how section, plenty of people willing to offer advice normally.
Good luck
ps.....first bit of self help knowledge would be how change a tube IMO. Then maybe setting the rear derailleur if having problems shifting, and take it from there.