Do things that show you are ready for the responsiblity of owning a pet.
They require things done daily: feeding, cleaning up after, water, play, ect.
So, ask for things to do around the house that show you can be responsible for a daily "chore". And do those things without being asked. If you have younger siblings, ask if there is something you can do to help them out. Help with homework, help them get dressed, things like that.
Saying "i promise to blah blah blah" mean nothing to your parents. You need to show them that you can take care of a pet.
You could also try things like keeping your room clean, making your bed every morning, saving money.
Just some ideas. Also, have a "serious" talk with your mom about why you want a kitten and what she would need/want from you to prove that your are serious and commited.
Good Luck!!!