Jason, there is no "grand theft auto" charge in Illinois. There are degrees of theft based on the value of the item stolen.
As a 15-year-old, you are facing a wide range of sentencing options. These options include fines, counseling, community service, electronic monitoring and probation on the low end, up to removal from your parents' home for placement in a juvenile detention center or boot camp or possibly jail or prison.
Which sentence the Court will decide on will, of course, depend on all of the facts of the offense you are said to have committed and what evidence supports the charge, but it can also depend on what a skilled defense attorney can work out for you with the prosecutor.
The laws in Illinois changed in 2018 and it is less likely now, perhaps, that you will be incarcerated, but the risk of jail time still exists for felony theft.