How do i know if my car needs a starter or altinator?

You can try a jump start or charge the batt , make sure terminals are tight . If head lights are nice and bright then it may be your starter , If there's a clicking sound then I'll assume the starter solenoid is getting power , once you get it to run you can put a multi meter on the batt and test for 14Volts+ to see if the alt is working .

If you have to ask that question, you need a mechanic to test the system. He'll find the problem.

starter starts the engine. alternator charges the battery