lets start off simple.

how old is the battery? have you tested it with a battery tester? one that puts a load on the battery to test the cca? parts stores do this for free.

next simple test is to test the battery power car off, should read 12 volts, no less. then start the car re test the power at the battery, it should be as high as 14 volts.

then of course the battery connections and grounds could be dirty or corroded. clean them up. wire brush does a nice job.