I need to change the battery on my car. Does the new battery have to be the same voltage as the old one? The old one is very weak and will be dead after the slightest thing.

Yes, you will find that almost all car batteries are 12 volts, though you can find some unique voltages out there (24V etc.) The biggest question is a matter of physical size and whether the poles are on the top or side. Stop by a shop, even Sears, Pep Boys, Napa, most places will even install it for free. Good luck

Not knowing what type car you have, If it's 12 volts it needs to stay 12volts unless you plan to change all the electronics. Normally 24 volts are for diesels, they use two 12volt batteries. Earlier built cars in the 60 had 6 volt batteries. Sounds like you need to go to a battery supplier like Gregisteredtrademark said.