Note: This uses a Mac. If you're a PC person, try and find someone who has a Mac to do this. It really doesn't take too long. Failing that, there's probably some good PC alternative or you could just use the included MP3s.
Step 1: Starting up
The solution for me is to use my MP3 player I already had and just make MP3s that are at 85 - 90 bpm so that there's a beat for each footstep for one foot. After running with that for a bit I switched to 170 - 180 bpm so that there's a beat for each foot.
If you're just starting out, you'll want to work up to 85 bpm gradually.
I'm using Garageband on my Mac to make this. It's included in the iLife suite.
So, let's get started! Fire up Garageband and make a new loop track.