Find the address for your school's admissions office. The easiest way to do this is to consult the school's admissions website. All colleges have a website these days (especially the reputable ones), and the address to the school's admissions office can be found on it with very little effort.
Address the envelope (front and center) to be sent to "Office of Admissions," then the name of the school on the next line, and then the address on the lines after. An example would be:
Office of Admissions
Major University
1500 College Road
Collegetown, NY USA
This will suffice for general admissions-related correspondence. If the letter must go to a specific person in the office, you can simply include their name first in the address, like so:
Ms. Susan Dean
Office of Admissions
Make sure to include any formal titles if applicable (such as Dean, Professor or Dr.).
Write your return address (using your name as it appears on your school documents) in the top left corner of the envelope.