Put a meter on the battery with everything off - should be around 12.5v
Now start the engine, you should see 14.4-14.7v

If you don't see the 12.5v with everything off, then suspect the battery.
If you don't see the 14+ volts when running, then suspect the alternator.

If the above 2 tests are OK,
with engine running, measure from the battery positive terminal to a GOOD engine ground,
if you see more than 1/2 v off from the battery positive to negative reading, suspect ground cable or battery cable clamp problems.