ault in NY while 17?

I am 17 and live in upstate New York...the other day my 14 year old sister was walking home and this kid she knows ( who is also 14 and apparently a total a$$ hole ) punches her in the face resulting cuts on the inside of her mouth, for no reason and walks away. So what would happen if i confront him and basically "beat him up" for what he did. What if this fight is not mutual. So basically i am asking what are the laws concerning my situation ( 17 year old aulting 14 year old in NY )