Although it may sound like a cop-out - honestly, this is sound advice...
You should consider reading the manuals - Gecko manual to fit the appropriate resistor to set motor current so they don't get too hot and also the setting of microsteps etc. and the Mach3 manual to set steps per unit, velocity and acceleration etc. in motor tuning.
All the information you need is there although I do appreciate that it is not necessarily that easy to understand at first read.
See how you get on, then come back if you have any specific questions and we will try to help.
Tweakie.