First off, you will need a program to actually make it in. If you just want to make one really quickly with a half-decent result, use paint for it, but I would still suggest getting Photoshop or Paint Shop Pro. PSP can be freely evaluated for 30 days, and it's an excellent tool for those who can't afford (or find a place to download ;p) Photoshop on.

Make an image with a decent size, about 4-5 times less height than its actual width. 350x80 pixels might for instance. Fill it out with some colors or effects of your choice, then save it as a .jpg or .gif. Upload it to a site, and then link to it from your signature.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram