Is it possible to move to NYC with an aunt or do I need a visa?

You CAN work up to 20 hours a week on campus only while here on a student visa...but that will not cover your costs. You must have a ton of money in the bank to prove you can pay the tuition and your living expenses to get a student visa. If you apply to a university and are accepted, their international students office will help you apply for the visa. NO...you cannot jus up and move to the U.S. to live with your aunt. that is called immigrating and requires an immigration visa...which you have a one in a million chance of ever getting. A student visa is your best chance of coming to the U.S. IF you have the money.

Yes you need a visa.
You can get a 90 tourist visa at age 18

For a student visa- you must apply to the schools, be accepted, prove you can pay and then apply for the student visa. the school is now your sponsor.
You will need to show bank statements with proof you have the funds to pay for tuition, room and board and health insurance, personal expenses and travel costs.

You can work on campus only- but there is no guarantee you will get a campus job or that it will give you 20 hours per week. You would make about $7/$10 per hour

You can check the tuition costs on any college's web site.

I would recommend checking out a "regular" university with a BFA in theater/music/dance. what ever performing art you are in to

In order to get a student visa, you must first be accepted to a school AND you must be able to pay for all your own expenses for the duration. You cannot work in the US on a student visa.

You could visit your aunt for up to 90 days, but you'd then have to return home.

Do not even THINK of moving here to attend college, unless you have been accepted. There are MILLIONS of kids every year who think they can move here, get into Juilliard or Manhattan or Mannes or NYU/Tisch, etc. and make it big. I have adjudicated thousands of music students in my career - and can tell you for a FACT - this is a one a ZILLION chance. There are plenty of places that you can read about what it takes to come to the US and go to a college and/or work - asking HERE to see if we have some kind of secret advice, or shortcuts for you, is futile. Good luck - but WE are not your answer.

You just asked this question (although you have now clarified Northern Ireland). Why not read the very good answers you received on your previous question regarding visas & working in the US.