Legal Question in Criminal Law in Tennessee

It sounds like you are saying:

"I've been arrested for shoplifting and I INTEND to seek pretrial diversion."

(Because if you are "using" -- already under the terms of -- a pretrial diversion, then your case has already been heard in court).

"How is that going to work?"

(The terms of pretrial diversion are negotiated between the defendant and the State. In general, there is supervised probation, payment of court costs, restitution, public service work, perhaps counseling, etc. At the end of the probation/diversion, if you have done everything you contracted to do, the charge is dismissed and removed from official records.)

"Do I need a public defender?"

(It is possible to act as your own attorney and negotiate a pretrial diversion with the district attorney, but then I suppose it is possible you could act as your own dentist and drill your own teeth. Settlement of a criminal charge is a serious matter with long term consequences, and you should obtain the assistance of a lawyer. As for whether that lawyer will be a public defender, that is not up to you, it is up to the judge. Public defenders do not accept or reject any client, they are appointed by the court to represent a qualified defendant).

"Can you fail your first drug test?"

(In short: no. Failing a drug test -- which is simply another way of saying 'breaking the law while on diversion' -- is an excellent way to cause the diversion to be terminated and a final judgment of conviction to be entered, with a subsequent sentencing hearing. As a general rule, your court dates and first report to a probation officer will be at least 30 days in the future, plenty of time to quit and clean up before your first drug screen, so there should be no excuse for testing positive for illegal drug use. But if there is a speedy settlement, within 30 days of the offense, often probation officers will give a defendant 30 days to 'clean up their act' before giving them a drug screen).