Lots of things to "look at" All time wasters when a simple voltmeter placed on the battery terminals will tell you what you need to know. You will need a 'helper" to try the starter while you monitor the battery voltage. If the battery voltage RIGHT ON THE BATTERY terminals themselves does not fall precipitously than the problem IS NOT all the connection, corrosion, ground terminals etc. The problem is further down the line. Keyswitch, starter relay, starter solenoid and most likely, a safety switch. BUT if the battery voltage falls like a rock, the problem is one or all of the connection problems as outlined by others. Why guess at these things when it only takes a few seconds to test and form a hypothesis of the actual problem? Get a voltmeter, and understand how to use it. One actual test that takes about 10 seconds will tell you more than all the poking, prodding, pulling and guessing in the world.