How do you convince your parents?

How do you convince them to what?

If what you want is something that a reasonable person would allow, then you might try pointing that out, along with reasons your parents should let you ... whatever it is you want.

However, ultimately, the decision is theirs. Once you're 18 you no longer have to care what their opinion is (assuming you're not living with them), but until then it doesn't matter how much you want something, if your parents say no you don't get it (with some exceptions ... if your parents are actually abusive the government may intervene, so if what you want is "enough food not to starve to death" or "not to have to live in the closet" then there may be some help for you).