Hi there Marrzy, even though you say you are 18, it does sound like you are still very young and when it comes to owning a pet you need to show you are responsible. Can you do that?
This means doing things for your parents without having them tell you. Respecting them and their rules.
Do you have a job? are you paying any board?
Who will pay for the food that your snake eats, the electricy bill and if needed vet bills.
If you have this already sorted, i would ask your parents to sit down with you to discuss the matter, say to them that after hearing you out and they still say no then you wont bring it up again, as long as they sit and hear you out .
tell them you are serious about buying a snake and before they say no suggest they come with you to a reptile place to veiw a few of them. Show them how you are going to pay for the snake and its enclosure, tell them exactly when it will be feeding and how often the snake will come out of its enclosure. I would even ask them why they are saying no, If they still say no then there is not much you can do until you move out of home.
remember to sit down and chat with them about it, it cannot be done when sitting watching tv, or doing the dishes etc. and becareful how you react, having a tantrum if they do say no is not acting responsible.
good luck
Danni