Mountain Bike Fix

How do I fix my mountain bike brakes?
My mountain bike’s back brake does not touch the rim of the wheel until I push really hard on it, and even then it doesn’t activate the full brake. So, I’ve been using the front brake instead, but of course using the front brake immediately can cause toppling over. So, I know I need to readjust the back brake.
So, what tools should I use to bend the brake pad? How should I align it to the rim of the wheel? Or should I just go to a bike shop? How much would it cost?
Thanks in advance.
Great thanks!
The adjustment is pretty simple, takes a couple of minutes and you can do it with a 5mm allen wrench, All you need to do is adjust the brake cable on the rear brake which has stretched. Watch this youtube video, it shows how.
http://www.youtube.com/watch?v=mGgidUE8drE
Mountain Biking Maintenance and Repair : Apply the Chainguide on a Mountain Bike

