1. Check that the battery connections are tight, clean and greased. The starter motor needs a high current, so it is essential that there is no resistance due to corrosion or looseness at the battery connections or in the wiring.
2. Check that the connections to the starter motor and battery selector switch are tight and clean for the same reasons as in 1.
3. Check the battery voltage, especially if the motor is trying to turn the engine but failing to do so. Use a voltmeter or multimeter. Alternatively, switch a cabin light on to give a rough idea of battery state.
4. Check the switch wiring. Starter switches often use push-on terminals. Check these for looseness or corrosion.
5. The starter motor may be jammed. Rotate the engine by hand to free, and try again.