My question involves criminal law for the state of: California

19 years old, caught stealing $110 of merchandise from Target in Contra Costa County. First offense, never done anything like this before. Had the intent of purchasing the items, I had enough money, but had a lapse in judgment once inside the store. I'm assuming any kind of motivation or excuse why I did it is irrelevant. I was told I would be sent a bill and a court date in the mail.
Called the Public Defenders office but they were very short with me/would not answer many questions.
Apparently a cop was already in the store when I was caught, and was in the security room. He didn't ask me any questions or do anything really but write down my name, just seemed like he was hanging out in there. How does this affect my situation in anyway?
If I were to take a theft class before going to my court date would this improve my chances at getting this off my record?
I cannot afford an attorney, how would I go about getting a public defender?
I've read online about diversion classes and ACD but am not aware of how they work or what makes you eligible for them.
I've also read that many people advise not to pay the civil bill that the store will send, why is this?
I've seen many other law advising forum claim that you should not plea guilty. I clearly committed the crime and was caught, should I plea not guilty? What happens if I were to do this?
I'm just a college student that made a stupid decision one day and I don't want it to haunt me forever. What are my options to keep/get this off my record?