Anytime I've tried to make a D3 build I've looked over all the abilities for a class to find the one I want to focus around due to it being powerful or fun (hopefully both). Since this game is fully built around having a resource builder and finisher I then pick out whatever move compliments what I already chose. Since I personally will be running solo mostly I then look for 1 or 2 options that would help my survivability the best. Fill in a resource dump somewhere, then I have that one last spot open for whatever works best with that build. Don't forget to review your passive abilities as well cause you can do builds completely based on those as well.