My name is Samantha as well, which may be partially why I'm actually responding but that's beside the point!
I've never gone by my full name, it's just....silly to me, and way to formal so I've always been Sam/sami/sammy. Like yourself the first legal choice is Samual(I like the a instead of e..just because.) due to ease of well just about everything, even my middle name will just be turned masculine "ann" into "anthony"... However, it's just not a name which /suits/ me. I go by Alder, everywhere to everyone. It's a name which was given to me by my brother about 10 years ago, and everyone says it suits me very well... however, I'm not sure how I would like it as my legal name since I feel it's a bit of a blow to my parents completely tossing the name they gave me away.
Being 17 I feel it's safe to assume you've not much time left in highschool? I wouldn't worry to much about paperwork for such things then, and spend the time finding a name in which you do like to be called, possibly ask your parents what your name could have been if you where born male bodied? It may help them as well feel more a part of your "new" self.