2005 Nissan Sentra My starter

2005 Nissan Sentra 4 cyl Front Wheel Drive Automatic

My car wont start. When I turn the key all the lights come on and the engine starts cranking up. Right as soon let go of the key the engine stops cranking and it turns silent. Someone told me it could be the alternater, the starter or the fuel injector. What could it be and how could I fix it.