I ran away from a group home when i was seventeen. I am now eighteen wanting to turn myself in, but i want to know...

You cannot bargain for any particular sentence until you surrender. The court can retain jurisdiction over you and detain you until age 21, however that is a very extreme remedy reserved for violent youths who have committed crimes like murder and need to be kept from society as long as possible. Because you committed no crimes while a fugitive and voluntarily surrendered, it is likely you won't be held much beyond your first court hearing. But no guarantees. Staying a fugitive will only make matters worse once you are caught.

Yout have to solve this. Being a fugitive from the law is no way to go on HIre a lawyer. Plan your surrender. Negotiate the best you can get