You need to perform a battery load test. If you're going to be using a Snap-On Battery Load testing device or some sort or even a VAT40, set the amps to 180. Your final voltage reading should be above 9.6volts. If its close to 9.6 volts then you might want to consider replacing your battery. If it's below 9.6 volts then you definitely need to replace the battery.

If replacing the battery fixes the problem the sad news is that the problem may reoccur because you might have a charging system problem. Or even worst a parasitic load. Meaning that you have a device that is draining the battery when the car is turned off. It also helps to make sure that the battery teminals are securely fastened onto the battery post. If there are any corrosion (blue powder) on the battery post then that means a bad or intermittent connection. You'll need to clean that up by brushing it away. Besure not to breath it or get any on your clothes 'cause it'll eat up your clothes.

You're problem may be as simple as an old battery and replacing it will fix the problem. But before you buy a new battery be sure to do some diagnostics so you don't waste your money on parts you didn't need.