What's the hardest part on becoming a police officer?

They will usually tell you why you didn't get hired. You should call and ask. Police jobs are very competitive. At my department we recently had about 2500 people apply for less than 100 positions. Most large departments like SDPD and LAPD can be very choosy. Go onto the california website for police standards and make sure your past minor transgressions don't disqualify you. Good luck!