I have a 1998 chrysler sebring I got from my cousin who said he put a new battery in the car but it won't start unless I jump it & I can't figure out what is the problem

The battery is no longer good, if a new battery was put in, then the charging system is not keeping it up.

Remove and fully charge the battery. Most auto parts stores will do this for free. Afterwards, have the autoparts store test your alternator for free.