On Nov 14, 6:58*pm, Death From Above
wrote:
I have a mongoose mountain bike with 3 gears in the front and 7 in the
back. *The rear derailleur is all bent up and needs to be replaced.

Is it easy to do? *Do I need special tools? What kind of rear
derailleur should I buy? *Are they any good cheap ones online?

You can find instructions he

http://parktool.com/repair/

Something like a $20 Shimano Altus will do you just fine. Just use
your search engine for the best deal.