Step 1: Make an .svg file of the shape you want to extrude
First get a shape that you want to extrude. It needs to be in a vector graphics format, not bitmap (or raster) format. Read up if you're not familiar with these terms.
To get a vector graphic you have three options:
Download it. You could use Google Advanced Image Search and limit your search to .svg files, a common vector graphics format on the web. You want to make sure you have permission before you use anyone else's drawings, though.
Make a digital trace. If you do have a raster image (bitmap), then you can convert it to a vector graphic with Illustrator's live trace or Inkscape's trace features, which are fun to play with. Keep in mind that these features are not magic, and you'll have to fiddle around a lot to get the graphic right.