How should I plead for a shoplifting, first-time offense?

Plead not guilty, and get a lawyer.  After you and your attorney review what happened, and whatever it is that you wrote out in the store, it might make sense to change your plea.  But it's much harder to change a plea from guilty to not guilty, than the other way around.

Your attorney should also try to get you into an alternate sentencing or diversion plan, as a first offender.  This is your best outcome (other being acquitted, which sounds unlikely).  This would mean you would be on probation (by another name, but the same thing, really), for about a year and a half, and if you complete the program successfully, the charges are dropped and your record goes back to being clean.  Plead not guilty tomorrow, if for no other reason than to give yourself the chance for something like this.