First, thank you for contributing!
For small things like typos, or trivial bug fixes, please feel free to submit a PR.
Code changes should be accompanied by tests where it makes sense.
For anything larger than that, please file an issue first describing the change you'd like to make so it can be discussed. We want to make sure your time is respected. If for whatever reason we decide a change can't be merged in, we'd prefer that is decided before you put a lot of effort into the change.