First impressions are not completely important, but it is the second impression that is the most important. First Impressions are only clues, not a means to determine the whole identity of someone.
It's your first day of college, you're a freshman again just like high school.
Just be yourself and let them come to you, then you can go from there. Don't rush into the dating scene right away, as it won't allow yourself to put your true self, and full potential energy and love into the relationship.
Wear something that shows off your personality, for example a band tee if you are into that sort of stuff. If not then just wear some black skinny jeans (trust me, girls find them hot) and like a basic t-shirt :)