Whenever I start to do something to one of my bikes that I am not sure about, I go to these websites first.

Sheldon Brown has more good info about anything to do with bicycles than anyone on the internet. About the most important link anyone interested in bicycling could have.
http://sheldonbrown.com/repair/index.html
http://sheldonbrown.com/home.html

Park Tools has an extremely helpful repair site. It shows you what tools you'll need and what to do with them. Just click on the part of the bike you want to work on.
http://www.parktool.com/repair/

If you are learning to maintain a bike, I suggest getting a good bike repair book in addition to the online resources. Park's Blue Book and Zinn and the Art of Road (or Mountain) Bike Maintenance are usually recommended.