8 months pregant & boyfriend walks out

If you can afford a lawyer, file for sole custody, child support kicks in pretty much at the same time considering the father signs the affidavit of Paternity; if your child's father doesn't sign an Affidavit Of Paternity when the baby is born, it will have to be done when you file for custody because the court needs proof that indeed he is the biological father. Once this is done ask your lawyer to state that he only has standard visitation.
Alot of lawyers don't charge for the first consult, start doing that now and see how much it will cost, THINK WITH YOU HEAD DO NOT THINK WITH YOUR HEART on this one, 18 years is a long time and you don't know what will happen, also, if you don't want your child to end up with your ex in the event something happens to you, once you have sole custody you ask your lawyer to submit to the court a Joint Conservatorship in which you name the person of your choosing to be your child's Conservator... guardian is not the same as conservator, a will and testament naming a guardian doesn't mean anything when it comes to minors and in the event of anything happening to you, the court will attempt to place the child with the father or any other relative unless there is a Joint Conservatorship filed in the court, with this your ex will retain only his visitation rights.