Roommate-assignment policies vary from college to college. Most colleges have a housing form that all freshmen-to-be who plan to live on campus must complete. Often this form will allow you to include the name of an intended roommate. In such cases, if you put your best friend’s name on this form and then your friend puts yours, you will be able to room together. (Note, however, that colleges commonly claim that they will “try” to honor your preference but can’t guarantee it. Yet, in most instances where specific roommate requests are permitted, you will get the roommate you chose.)
There are also many colleges that do not allow students to pick a roommate. The college administrators believe that part of the college adventure is meeting new people and learning to co-exist with someone you don’t already know.