Are you sure that ALL other electrical components work okay? Have you put a voltmeter across the battery to see if there is anything at all in it?

If the headlights come on brightly, then for the short term consider the battery okay and measure voltage at the input to the starter solenoid while someone turns the key. If you have 11 volts or so at the solenoid when the key is turned, then the problem is the starter.

If your headlights are weak, have the battery load tested. Most anyone who sells batteries will do this for no fee.

If you have NO voltage at the solenoid then check your neutral safety switch and back track from there.

Good luck,