Most likely the battery! so:
charge the battery, even though inside lights and other stuff might work, the battery may be giving up. or put it on jumper cables, and see if that fixes it. You always take the battery in to any number of autoparts stores, and they can put it on a tester to see if it is good, and if so put a quick charge on it.

Eliminate any possibility of it being the battery before you move on!

Start with the most simple most direct diagnostics, eliminate the obvious before you start theorizing!