FFBBT: im just a young kid without much experience in the world, and therefore, while i wish you the greatest hazlacha in life, i dont really have any good advice.
however, im a little confused about your situation. are you thinking about going to seminary next year? if so that would put you in 12th grade. that seems a little early to get married, unless you are now chassidish (which is not the way your story sounds). and even if you think you are ready to get married now, how to you know this boy if your in 12th grade? the only possibility i can think of is that he, like you, moved over from left to right, and you know him from your MO days.
if you already the normal age to get married, wouldnt it be to late to go to sem?
whatever age you are, are you sure this boy would marry you? are you sure hes ready to get married? have you/are you dated/dating him?
have you discussed marriage wit your parents yet?