I'm assuming that you mean you need to jump your car to be able to go get your voltmeter or to crank the car.
You need to check the voltage of the battery without the car running first. You will get the most accurate reading by checking at the battery posts, but the hot post under the hood should give you virtually the same number. No need to check it at the battery for this test.
If the battery is flat (meaning less than 12.6v), then you need to charge it by using a battery charger. Let it charge for a few hours. Here's my recommendation. Check the battery voltage immediately after charging. It should be 12.6v+. If not, then the battery will not even take a charge and is trash.
Check it a few hours after charging. It it drains much after charging, then either the battery is bad or you have a parasitic draw. If it holds a charge, then crank the car and check the battery voltage at idle. It should read 13.8v+ if the alternator is functioning properly. If it is less than that, then the alternator is almost certainly bad.
If all this is ok, then take it to Autozone (or the like) and have it tested under load and have the amperage tested.
I think that your battery is toast or the alternator is toast or both. Genius huh?
Anyway, it shouldn't take you long to sort this out.