Agda and its related libraries are hosted at Github. To contribute, you will need to fork a repository, make the changes and then send a pull request (PR).
A code of conduct and other considerations are described in the HACKING.md file in the root of the Agda repository.
You can also take a look at the current Agda issues to help us solve them. You can start with the label difficulty: easy and help wanted. You can also explore all the labels.
The Agda User Manual is a work-in-progress and is still incomplete. Contributions, additions, and corrections to the Agda manual are greatly appreciated.