The first thing to do is to clean all of the connections to and from your battery. You can start with the battery terminals themselves, and then follow the battery cables, inspecting and cleaning everyplace those cables go. Do the same from the alternator out the other way. Make sure that one of these paths hits the connections at the starter.
If all of that gets cleaned up and the problem persists you’ll want to have a multimeter and actually check the cables (they can be internally corroded), and have someone test the starter/solenoid.