I will take a stab at this....
rwarre, you are not giving a lot of info on this one but let's start with the battery. Take a volt meter and verify you have twelve volts across the battery terminals (the positive and negative terminal).
I suspect you are using a solenoid between the battery and the starter. If so are you getting twelve volts between the solenoid battery terminal and ground? Make sure the master solenoid is activated or turned on before testing.
Now, let's assume the above was ok. You will need to connect the volt meter or a test light to the sol terminal on the starter solenoid then turn the ignition all the way to the crank (spring loaded) position and note if the test light or volt meter register 12 volts. If so, then the switch is probably wired up correctly. The next step is to make sure when the starter switched is turned to the cranking position, the starter terminal on the starting solenoid should have twelve volts. If not the solenoid may be bad. If it has voltage on all three terminals while in the cranking position, you may have a faulty starter (usually a bad spot on the commutator on the armature) .
Also make sure you have a good ground on the engine case and airframe.
Hope this help.