You need to start with some basic checks first like checking to see if you have a live feed to starter solenoid wire, then checking the main cable from the battery to the starter and checking all earths.

Does it make a sound when you turn the key?

If you get a feed down the solenoid wire but still nothing happening (solenoid should click loudly at very least), it doesn't mean that all is well the wiring as you may have high resistance. Try putting a seperate live feed to the solenoid and see what happens.