I'd begin with cleaning & tightening the battery terminals. And the other end of all the cables. That crusty green crud can sneak up on you. Disconnect (-) so you don't short to ground with a wrench.

Then if that doesn't help...
When it does NOT start, have someone hold the key at the START position while you hit the starter with a broomstick. If that makes it start, then you need a new starter. Or rebuild it by putting new brushes in & new contacts in the starter solenoid.