You can start with Pronterface, this is what almost all people do. But I would start with Repetier-Host ( I use Repetier-Host-Mac).
I downloaded the source too and I'm running Repetier-Host in debug mode. So far I only discovered a bug, which Repetier fixed in a short time.
You need also a slicer-software. Repetier-Host's default is the internal installed slic3r, which is quite easy and fast.
That means, you can open a stl-file and generate the G-Code inside Repetier-Host.
Selecting Printer->Printer Settings->Connection->Port: "Virtual printer" let you start learning Repetier-Host without a 3D-Printer -> it’s great isn’t it?
Our heads are round so our thoughts can change direction.
Francis Picabia (1879-1953)