If you've never been in trouble with juvenile court before, you can expect a letter from the diversion unit offering you a diversion. That means if you accept the diversion offer, you will do some things like community service hours, theft reflections classes, etc. If you successfully complete the obligations, the diversion is closed and you never go to court.
If you do not get a diversion, and go to court on the charge, the standard range sentence is 0-30 days detention, 0-150 hours community service, 0-500 dollar fine and 0-12 months probation. If your area has a "teen court" you could be referred to it. That is where your peer teenagers will hear and argue your case and the teen jury will decide your fate.
A little extra information: If you were trespassed from JC Pennys or the mall, be sure to stay away. If you enter the store or mall (if you were trespassed from it) and they even think you are there to commit a crime, like shoplifting, then you could be charged with a Second Degree Burglary, a class B felony.
You don't have to worry about the worst case scnario because with no history you won't get the worst. You will be given the oportunity to go through a diversion and if they have a good case against you for theft, then you should do the diversion. If they don't then you should fight the theft charge. The only way to know for sure is to have an attorney read over the police reports and see who is saying what about whom.