I just put in a new starter and it still won't.

That's not a starter problem. That's low voltage due to a bad battery or a loose or dirty connection. Start by measuring the battery voltage. It will be 12.6 volts if it's fully charged. If you find around 12.2 volts, it's okay but discharged. Charge it at a slow rate for an hour, then measure it again. If you find closer to 11 volts, the battery has a shorted cell and must be replaced.