You could start by making your own local copy of the repository by cloning it with Git and committing your changes to your local repository as you work on it. When you have some changes to share, you could make a publicly available clone on https://rcs.pfsense.org/ (which is where the main pfSense repositories also reside) and push your changes. For testing your changes, a system (either physical or virtual) with a full install would be useful so you could use gitsync to apply your changes to your test system (playback gitsync from the developer shell on the console menu).
Some useful documents for a new pfSense developer:
http://devwiki.pfsense.org/DeveloperRules
http://devwiki.pfsense.org/GitWorkflow
If you do any development, it should be done on recent 2.0 snapshot builds (except if you are trying to make a package for 1.2.3).