Umbraco is Open Source which means you can contribute to make it great! The following pages provide you information on how you can contribute to different parts of the project. Remember that contribution is not just 'code' but can also be constructive discussion and contribution to documentation.
The documentation below is very very detailed. We're a friendly bunch however and want you to be successful without bureaucracy. If you're just looking for a quick guide on how to get started then there's the following short steps from the "Quick start guide":
Read the "Quick start guide" first and then when you've done your first few pull requests you can dive into the nitty gritty details of the other pages here. It's easy to get started and we will help and guide you every step of the way.