Today I was caught shoplifting a total of $126 in items from walmart. I was completely cooperative with the security guard and completely honest. The security guard said she was not going to press trespassing charges seeing as I cooperated. The cop gave me a notice of referral for possible pre-file diversion. I'm not exactly sure what any of this means because I have never been in a situation like this. First time I've been in trouble with the cops, first time I shop lifted, and I regret every bit of it.

SO what is going to happen to me most likely?

Any question on law needs to include your general locale as laws vary by area.

You refer to a security guard and a cop. If by cop you mean an officer of the local police force or sheriff's dept, then charges are being pressed. The store security would not have called the police if they weren't going to press charges.

Diversion is a term I've seem mostly associated with Canada. It refers to a first time offender who is put on probation for a crime. If they fulfill the terms of the probation satisfactorily, then the charges are dropped. But since we don't know where you are, we can't be sure that is what was referred to.

If the security guard did not call in local police, then the likelihood is you will get a letter from Walmart charging you a fine. Usually $300-$500. Upon payment of the fine, no further action is taken and you will not have a criminal record.

... The security guard said she was not going to press trespassing charges ...

If this is a first-time situation as OP indicates, I don't know what trespassing would have to do with it. Stores such as Wal-Mart sometimes impose a condition that the culprit not come back. If they do, they could be prosecuted for trespassing.

I've heard of some areas prosecuting shoplifting under trespassing laws.