Hi, you need to check your battery is charged fully and is being fully charged by the alternator if the alt is not charging the battery properly then this wont give you enough power to get the starter motor working so your best option is to buy a battery charger and connect this to your battery and give it a full charge.
Once its fully charged then try starting your car if it then starts you then know you have either a faulty alternator or faulty battery, to test this you need to make sure the alternator is putting out about 14volts to the battery to ensure its being charged properly.

if the alt is putting out 14volts then the chances are the battery has got bad cells and you will then need to get a new battery.

you need to make sure you have tested the starter motor with a fully charged battery so getting this charged would be the next thing you need to get done.

let me know how you get on or if you need further assistance ok