Reading!

As a Government major and Environmental Studies minor, I have to do a lot of reading. College everywhere is different from high school in a number of ways, but the most difficult part of the transition for me was learning how to accomplish all the reading my professors expected. You will have to read more than you thought possible in college. Whereas in high school you have to read a chapter a night, in college, there is a strong likelihood you will have to read the whole book for the next class. Not to scare you or anything, but it's a lot of reading. Luckily, if you know this coming in, you can prepare yourself. Learn how to skim effectively. Dartmouth also offers a great speed reading course which a friend of mine recently took that helped him speed up significantly.

So be ready, but don't be scared. In college you get to take classes that interest you, so the reading isn't bad. It's a lot easier to read a book when its in a subject you love.

More info on academic resources here.