First, you can go to "Thingiverse" website to download what stuff you want. This website has many .stl CAD file.
I put .stl file into "Slic3r" to generate G-code, which means the command we ask machine how to move. For example, G01 X32, means "move to x=32 position"
Please refer wiki/G-code. Of course, the printer couldn't support all codes, please check this point.
And then, I load this G-code into ReplicatorG, which is the software could drive my mother board. Finally, let it to start printing.