A Dead Battery
Symptoms: Your car won't start
If you don't have a tester or charger at your house, places like Autozone will check it for free to make sure the battery is actually the problem and not the starter or alternator. If there's any corrosion, clean that off with a metal brush. Unbolt the negative terminal first, then the positive. There might also be a ground wire that you'll need to take off. Then, put the new one in. It's that simple.