Third degree theft, paid 200 in fine at the store for something worth $50. What to expect in the court

You did not pay a 200 dollar "fine" if you have not been to court, you paid the store a 200 dollar penalty. You can expect another actual fine in court

Sir, what can I expect in court. I am 15 year old 4.0 student in 10th grade

Best guess, judge will talk to your parents, check your school records, again it is only a guess, but a fine of 100 to 300 dollars, perhaps community service with a short probation time.