If your battery is faulty then this will mean that it is charged by the alternator, but that it does not actually hold on to that charge.
The first step to diagnosing this problem then is to try jump starting your battery. This is necessary to charge it again at least briefly and this will help you to diagnose the problem. To do this you will need the engine from another car and two jump leads. Simply connect the positive terminal on your battery to the positive terminal on another car, and then connect the negative on theirs and ground it on your car by clamping it anywhere on your engine that is made from metal. Now ask the owner of the other car to start their engine and keep it running for around ten minutes. After this time you should then be able to start your own engine. Keep both sides running and then after a few minutes disconnect both without turning off your engine. Now when you disconnect the batteries make sure to go for a short drive.