This is my first violation of probation for failing a urine test and am on first offender act. What can I expect?

You may be in trouble, hopefully the PO will give you another chance but Rockdale country is sort of tough on these things.

So
1. give you one more chance
2. give you some jail time and back on probation
3. give you a lot of jail time

You will most likely love the first offender act and the original crime will now not be explunged at the end of your probation.