The first day is the first appearance you have - make it a good one, because first impressions matter.
Keep things simple and classic, nothing in-your-face or over the top. It's okay to be a little trendy, but you have to adapt the trends to your personal style, or you'll end up looking like a major trend-chaser.
Put on the confidence! If you don't have it, fake it. When you aren't doubting yourself, no one else will doubt you or your clothing choices.
The idea behind a great first-day-of-colloge is a killer ensemble. Make sure you can back up your outfit & will keep your dressing pattern consistent - don't wear an ultra trendy, right-off-the-runway look and then turn around and wear simple jeans & a tee combo the rest of the year.
Look good, but nothing too dressy or attention-grabbing.
So get going!!