I am 17 years old and pregnant in dcf custody. When I turn 18 can I walk out of the system as long as me and my baby test negtive for drugs and I have somewhere to go?

You may be allowed to leave but DCF will wait for the baby to be born and may get a court order to take the baby as your unfit to be a Mother.  There are several scenarios that could be played out here for this and letting you and your baby walk away free could be one.  Good luck