If you have improvements to Viglet Shio CMS, send us your pull requests! For those just getting started, Github has a howto.
If you want to contribute but you're not sure where to start, take a look at the issues with the "contributions welcome" label. These are issues that we believe are particularly well suited for outside contributions, often because we probably won't get to them right now. If you decide to start on an issue, leave a comment so that other people know that you're working on it. If you want to help out, but not alone, use the issue comment thread to coordinate.