I don't know how old you are or the reasons why your parents are not keen on the idea - as others have suggested it may be useful to you to discuss this with them and go from there. Don't argue initially, just discuss openly and try to understand their point of view.
I agreed for my daughter to have a lizard and 2 years later a snake. She is only 10. I can tell you about the things that went on to convince me that it is reasonable for her to have these pets, as a parent.
1) Commitment.
If we think it's a phase, or you are a kid that develops different interests regularly the concern is that it will be an interesting novelty at first and then the phase will pass. Borrow and read lots of books about reptiles and care in captivity, make a list of what you need to know about equipment set up and ongoing care, handling etc. But better still if it is something you have shown commitment to over time. Join the local herp society, attend meetings and contribute and even volunteer for stuff if you can. (For my daughter I'm talking years here)
2) Responsibility.
We need to be confident you can care for another being for its life. Say you do your regular chores that you're supposed to, are fairly reliable in the household, and have an understanding of what animals need and the interest to deliver. ie You will prepare the food, change the water bowl, clean out the enclosure, etc without us nagging or having to do it ourselves because you don't want to anymore.
3) It's our problem.
Ultimately, responsibility (and cost) for the animal falls back to us as the adults. So in the end, once you move out/go overseas/go to uni/lose interest or go do something else it's possible (likely) we will end up with it for 30 years. So we need to be ok with that possibility and comfortable with the animal and set up. Also it goes in our house, it needs equipment, it needs to be set up somewhere and it may need holiday care or whatever. If they just don't like it or aren't prepared to deal with this there's probably not much that can be done to change their minds unless the first 2 points are so convincing that it outweighs this one, and even peaks their interest enough to bring them around to the beauty of reptiles.
These are my thoughts as a parent, not sure if it helps you, but decided to add as it might give some ideas as to parents make these types of decisions - no, it's (usually) not just because we are mean dream squashers!
ADDED: And definitely don't do anything that would show disrespect, immaturity or get your parents offside, like get it anyway or hide it. Best thing for the snake is to have parents on board as you will probably need their support and resources for SOMETHING at some point. Acting like a turd will definitely not help you and will only show you aren't mature enough to have a snake.
