How do I contribute to Xapian?
There are a couple of useful documents for someone wanting to start contributing to Xapian.
Firstly, there's a guide for potential GSoC students, which should take you from nothing up to being able to work with the Xapian codebase, and also has some suggestions on how to think about a project proposal (even if you aren't considering applying for GSoC yourself, larger pieces of work are worth planning carefully first, and often getting feedback from the community is helpful in getting you pointed in the right direction before you begin coding). Secondly, we have a HelpWanted page which suggests various things to tackle, including tasks that don't involve coding, such as improving our documentation.