Mrs. Chip always laughs at me when I say this, but I really mean it... the only transition that's harder than high school to university is kindergarten to grade 1. Don't party too much, but don't study too much either. Party at least a little, but study at least a little too.
And for goodness sake, whatever you do... don't fall into the trap of thinking you're too busy for a girlfriend. If the two of you can get through university together you'll last long after.
Edit: Know what I got when I graduated high school? A job... Jeez, kids these days.
Good point. It is important to party, in moderation. I ended up marrying my high school sweetheart while in college, and I couldn't imagine having a better "mate". While that's not the right move for everyone, it was great for us. We both handled our business in the classroom and out. It was great. I found the biggest key to performing well in college was to attend EVERY CLASS.