To everyone who thinks the OP is immature,
Please give him/her a break on this one. It's only normal for someone who's going for his/her first pet to act immature-ish. I think that most people will botch their first pet no matter what their age is.
But that doesn't mean that they shouldn't get a pet. Taking care of a pet really isn't about 'responsibility' or 'expenses'. It's all about experience.
And yes, I'm speaking from experience here. Having a pet is like having those tamagochi pets where if you forget to feed it, it'll die (and you'll have to ask your parents for another). The difference is, real life pets aren't likely to starve to death. Instead, they're going to become more and more suicidal (usually by refusing to EAT, yeah, responsibility...) and then die because of some sort of disease (or accidents). This is... somewhat INEVITABLE.
Over time, you'll realize that a pet can get mental disorders and unless you'd decide to go to animal psychology, you can only learn the signs by experience (or online guides, but don't bet on them). Either way, always think that pets are by default, suicidal. It'll save you and your pet's life one day.
Also, cats are evil. Unless you're evil too, it'll be hard to take care of a cat.