There are a number of possible causes: bad battery, cables, cable connections, starter solonoid, starter, wiring, ignition switch. To determine correct cause it is best to diagnose when the problem occurs. Check battery cables at both ends to be in good shape and tight. With problem present check voltage while trying to crank at: Battery posts, battery terminals, large cable at starter, small wire spade connector at starter. The results will narrow down the possibities.

Another approach is to run a small jumper wire from battery positive and touch to the small spade connector at starter. If starter suddenly works then you have eliminated starter, battery, cables, solonoid and starter as possibilities. This would also get ou on your way. The problem then would be ignition switch, associated wiring including wire to the small spade terminal at starter.

Good luck