How to tell degree of trespassing charge?

I got charged with trespassing and received a yellow ticket with a court date. Under the section where they write what crime I committed, it just says 'trespassing' there is no degree or anything. The officer did not tell me which degree either, and at the time, I didn't know there were two types, so I didn't ask. He simply said that 'next time it happened, I would go to jail'. Should I expect it to be a 1st or 2nd degree trespassing? I was in an area of Walmart parking lot and honestly had no idea I was trespassing. I have no problem paying a fine, but I really don't want to go to jail for something I didn't know was illegal. I can't afford a lawyer and my local court says they do not provide lawyers (yes, I asked).

Also, would it be best to simply plead guilty or try to plead not guilty and defend myself in court? I know very little about the legal system, since I have never been arrested or anything before. I had a shoplifting charge, but it was brought down to a littering charge and I paid the fine. Other than that, the only other law violation was a minor traffic citation.