Learning to fix bikes is like learning to swim(?), nah, that's boring. Let's say it's like learning how to make babies. It starts with gettin' dirty.

Get an old bike (with derailures like a 10-speed) and a bicycle maintanance manual. Take the whole bike apart and put it back together. I mean take the WHOLE bike apart. Take SOME spokes out off the rim, take all the bearings off. Take the cables out of the casings on the shifters. Take it ALLLLLLLLL apart.

Then, put it back together.

Then, ride your re-assembled bike 100 miles and see how it holds up. Hopefully something will go wrong and you will know how to find it and how to fix it.

Offer to tune up the neighbors bikes.

Ride other people's bikes that do not maintain their machines and see if you can find out what is wrong with it (if anything).

Take good notes as you take the bikes apart. It makes getting them back together easier.

Once you think you are pretty good, ask you LBS if you can work as a wrench helper for a couple of days. You will learn about a variety of different components and will make friends.