help D: read desc

To make the deal rock solid after bothering them non stop
Make a inescapable contract which no loop holes can be found and make them sign
Keep this contract somewhere secret where they'll never find it (like inside a small box inside a drawer)
When the time comes to get a cat, they might try to back out in which you will show them the signed contract which legally bonds them to getting a cat
If they do not comply well you have a future case to take to court
(That last step can probably be ignored)

Ok this plan is basically fool proof unless someone is allergic
Go up to your parents and ask why they won't let you get a cat
If it's because of your age then make a deal that when you turn 13 then you can get a cat
If it's because of who will take care of the cat and do this and that
Then say you will that comes with you keeping up your side of the deal of feeding it, watering it, playing with it, and cleaning the litter box
It can be hard and absolutely stink but after like half a year they'll probably ease off it a little and help you do the stuff
The last possibility could be school
This one is hard to get around but it's possible
You have to become the quiet kid in the class and do your work and homework on time
It helps to be quiet because the teacher won't bug you as much

All over*

Tell them cats don’t take much to take care of, and depending on the cat, they’ll snuggle with you till you get up! :3
Also tell them they won’t jump all of you as dogs do. They’re pretty funny as well at points and make great companions ;]