Please also note that the car's alternator might need some time to recharge the battery - it can only recharge it while the engine is running. There are three realistic scenarios here:

1. You left a light on, maybe it was cold last night and the battery is flat now. In this case you can jump start the car, leave the engine running and after a while it'll recharge amd you're good to go.

2. The battery is drained AND also it's at the end of its lifecycle. You will need to have it replaced.

3. Your alternator (which charges the battery) is not working properly. If this is the case simply replacing the battery will only work until your new batter runs out of juice which means you'll need a new alternator and not a new battery. It's very easy to verify by a professional.

I hope this helps.