A pic would help to see how bad it is...For a mildly bent frame.

But what you will need is a wheel axle, hack saw, and some nuts for the wheel axle. And something to bend it with like a crescent wrench.

1) You cut the wheel axle in half.
2) Bolt each side of the axles on the drop outs so that they are facing each other.
3) Bend the frame till both axles are perfectly facing each other.