First you have to find a project you would like to work on. After that it usually goes like:

- Adding New Features

Is it a new feature I want to add to the project? If yes, find out if someone is already doing this feature, and then you have the option to work with them, otherwise, start coding and submit a pull request (or something similar).

- Fixing Bugs

Is it a bug you found while working with the project? If it is, then this is a good time to go inside the code and fix it and do the same pull request cycle.

- Performance Fixes

If you're a curious guy like me, you can go ahead and look through the source code and find places where you think performance can be improved, then start refactoring, after you're done, make sure you publish benchmarks in comparison to the old results.

Have fun coding! :-)