When I start my car up it voltage is reading.

Start by checking the serpentine belt tension. (Best to do that with the engine not running)! Next, measure the battery voltage before starting the engine, then again after it's running. Watch what that voltage does for about the first minute. Do this with a digital voltmeter, not the dash gauge.

If the voltage with the engine running is not between 13.75 and 14.75 volts, we need to look at the charging system. Holler back with those voltages.