Docs are a good place to start. In the projects I've worked on, new contributors usually enter when they've found a bug/desired-feature and worked out a fix for themselves. Aside from that, good first commits are usually smaller, so it's easier to review, and address a single concern. After you get a commit accepted, you can start claiming commits.

Answering questions on Stack Overflow / mailing lists / issue trackers can be a good way to build up trust in your abilities and a working relationship with other contributors.

Often projects label things that are "good for a first commit" and you should check out those.

All that said, most projects are eager to include new people, and personally I enjoy helping new contributors feel comfortable and get their commits accepted.