The first step would be to get the basic X and Y movement working, then the heaters and a single extruder, building it up slowly as you go.
You can use the web configurator to get a basic starting config, then you can manually add the additional axis and features from there.
https://configtool.reprapfirmware.org/Start
It may be a good idea to poke around the documentation for a bit and get a feel for what's possible.
And come back here to the forum when you have a specific question.