Thanks for your interest in contributing to Handel. There are several areas to contribute to.
For bug fixes open an issue describing the bug.
To add your fix, describe your proposed fix and make a pull request.
To propose a new language feature, open an issue describing the feature to be added. Be as detailed as possible, perhaps including examples of the syntax, etc.
Before beginning work on a new language feature. Propose the issue (as above) -- and state that you would like to implement or partially implement the feature.
Wait for feedback on the language feature, and whether it would make sense given the scope of Handel, before implementing. Then implement, and make a pull request.
This is to limit rejected pull requests, so your time is not wasted.
Creating tutorials on any platform is always welcome.