Don't worry, they'll be sending you a thick packet in the mail soon. Welcome to Stanford!
"Freshmen are sent a housing preference form in a booklet called Approaching Stanford in May after they accept their offer of admission. The housing form should be filled out online or returned along with other enclosed preference forms (for required classes, an advisor, and meal plan) by mid-June. Assignments are made in random order after the deadline to return the forms, not on a first-come, first-served basis."
Freshmen Housing