If you rode a bike before your amputation, you can ride again!  Depending on how serious a rider you want to be, there are several things you can do to make that happen. The biggest issue an amputee encounters when attempting to ride a bike is that it’s difficult to keep the prosthetic foot on the pedal.  The simplest fix is one I read about on the website of a former police officer who lost his leg above the knee (it’s the one I use).  You just wrap Velcro around the bike pedal and attach a large piece to the sole of shoe.  Mount the bike by standing next to it and swinging your prosthetic leg over the seat. Place your prosthetic foot firmly on the pedal, push off with your sound foot, and you’re off!  Your foot should stay securely on the pedal even when you encounter slight inclines and small bumps in the road. 
Some amputees (mostly BK’s) prefer to put a strap or cage on the pedal. The problem I’ve encountered with that (I’m AK) is that the heel of my foot tends to turn inward and hit against the bike. You could solve that problem by attaching a pedal extender.
For serious bikers who like to travel off of paved roads and bike paths, check out the following link:
Happy trails!