First, to answer your question, if you are found guilty of misdemeanor theft the question of jail will be left to the court's ultimate decision. Seeing that you are young and this is your first offense, while you can hope to trade on a court's expected leniency, I would suggest there are significant ramifications for your future outside of the jail. I would suggest you obtain a lawyer to help you defend this case. While it is a small amount, a crimininal conviction at your age would be terribly limiting to your career prospects. You need to have someone attempt to negotiate some sort of non-criminal resolution that could include the payment of the money/merchandise taken, and some other steps to avoid a conviction. If that fails, you need to position yourself to ask the court for expunction of your conviction at some point (because you are under 25 and may qualify for it under 973.015). That is the last resort, as amendment to a non-criminal non-descript violation or an outright dismissal is preferable.
In conclusion, I would seek counsel. If you cannot afford one, attempt to contact the State Public Defender's office to get assistance. Or ask the Court to appoint a lawyer for you. Do not try to resolve it on your own. You have too much on the line to do it.
Many times this will depend on whether you can make prompt restitution. If so, then you could probably expect a diversion or expungement. In any case you should be eligible for expungement if you have no other record. Very rarely do I see people serving jail on these unless they have a significant history. See what the DA has in mind and if he or she wants a criminal conviction out of this, get a lawyer.
I agree with the first answer. Theft of movable property is a Class A misdemeanor, punishable by up to 9 months jail and/or a $10K. You do not have to go to jail, but jail is a possibility. And, a conviction will have a significant collateral impact given your young age.
In Milwaukee, you may be able to obtain a deferred prosecution agreement. In other words, you would be placed on a sort of informal probation. If you successfully completed the informal probation, the charge would be dismissed or reduced to a civil violation.
Contact local, experienced criminal defense attorneys directly. Most initial consultations are free. So, you have nothing to lose at first. If you cannot afford a private attorney, see if you qualify for public defender assistance. If you do not qualify for public defender assistance, ask the court to appoint a low-cost attorney to your case.