The best place to start getting your feet wet is at our https://github.com/fritzing/. fritzing-app contains the code for the fritzing desktop software.
Get a quick start by reading the https://github.com/fritzing/fritzing-app/wiki. It explains how to setup your development environment and also something about the architecture of fritzing.
If you want to get your feet wet, you might want to browse the https://github.com/fritzing/fritzing-app/issues. Find a small problem that someone has reported and work on it to get a sense if it suits you.