Im 15 and I really want to get my belly button pierced. My mom thinks its cute but my dad doesn't want guys to look at my belly. My sister is 19 years old and I thought the law was that you had to be 18 to get it on your own with ID and 21 to sign the papers for anyone under 18. A girl at school told me my 19 year old sister can take me. If its my sister and she's 19 years old does she count as my guardian? And would she be aloud to sign the papers? I mean, what if I didn't have any parents? Does that make her my guardian if I live with only her? Anyway, I really want to get my belly button pierced and I'm willing to do anything to get it done. Can my 19 year old sister sign the paper?

I think it depends on where you live.
I live in cali and you need a parent or LEGAL guardian with you if you are under 18. You can't just get away with saying you live with your sister because they usually ask for paper work saying she is your legal guardian. And also at 16 in cali you can get any ear piercings with our your parents permission so maybe wait a year and get one of those :)

Okay thanks. I live in PA do you know if it's any different?she said she would take me to a tattoo shop to get it peirced but Idk if they would allow it.

The legal age is in PA is still18 but you can get a piercing at 16 if you have a parent or LEGAL guardian's signature. In PA minors under the age of 16 can get pierced if you have a physicians note allowing the pierce to be done. The parent must bring identification that is acceptable by the list above. The minor must have a school photo ID or a birth certificate with photo ID.
So in your case either way you need a parent until your 18... unless you make friends with a piercer :)
But whatever you do Don't PIERCE YOURSELF!! You could do so much damage that way.

I was considering that but I've herd bad things about doing it yourself.

Yeah don't do it yourself and don't have a friend do it, there are some piercings I would say sure do it yourself but not this one, there are so many things that could go wrong and belly piercings are difficult in the first place.