The battery starts the car. Then the alternator re-charges the battery. The battery should have 12V (+ a tad) available before you attempt to start the engine.It is possible your alternator is shagged. Get the engine running, then measure voltage at the battery - it should be around 14V. If it isn't, the alternator is shagged.
If the alternator is still ok, increase engine revs to around 2000, and measure volts at the battery - should still be around 14V. If it is more, the regulator in the alternator is shagged.