Two things: Load test the battery. To do this put a voltmeter set to dc volts on battery terminals and while cranking make sure the volts do not drop below 9 volts. If the voltage falls to 9 or below, clean the battery connections or replace the battery as needed.
If the battery is good, remove the metal cover and then the four screws and spring washers holding the control panel down. Lift up the control panel and you will see a black relay with 4 wires on it.
Leave the smaller blue and black wires (0 & 56) attached. Turn off the gas or pull the plug wires.
Pull the other two wires off and connect them together for a couple of seconds (with some metallic object, paper clip, jumper, whatever). The engine should crank. If it does not crank, then it is the battery or starter. If it does crank, then it is the relay.
When the controller wants to run the starter, it applies 12 volts to wire 56 which will pull that relay in which will energize the starter's solenoid. 0 & 56 is the relay's coil, and the other two wires is the wire to the solenoid.
State your problem, not your diagnosis.