Pair for a fortnight with someone who already knows the code. Have someone take you through the whole lifecycle of a few typical interactions between your project and the rest of the world. That's what got me started on the huge project that I currently work on. It only got me started though, it was at least three months before I was comfortable enough with the codebase to have a good idea of what each of the major building blocks does. During that time, I fixed little bugs all over the place, read the tests, re-factored stuff.
If it's a small project, read the docs, read the code, and read the tests. Then pick something to fix or improve.