Well, since the sentence is like saying it in a form that you are really happy, It would be the ! (or exclamation point) because the person in the sentence sounds really excited that they won.
I just won a new bicycle! (?) is asking a question and (,) is put in a middle of a sentence when you break the sentence from 2 into 1 sentence. It is (!) because you are excited that you won a new bike. A sentence always ends with punctuation.
