Most of you will hold office hours, and it is advisable to determine two things ahead of time: (1) realistically, how many hours can you devote to office hours? and (2) how many of those hours will you be available on a walk-in basis, versus by appointment? Initially, you might try having at least a few walk-in hours per week, since many students, especially freshmen, seem to be less inhibited about stopping in for help at a designated hour than about asking for an appointment.
In addition, you will need to decide how you want to schedule those office hours. Since courses in the College run on Monday-Wednesday-Friday or Tuesday-Thursday schedules, it is better to split your office hours between those two subdivisions of the week. For example, you might have your office hours on Monday and Thursday rather than on Monday and Wednesday. By splitting your schedule in this way, you become available to the largest possible group of students and thus will probably have to make fewer individual appointments.
Read more about office hours in Stanford's online handbook (PDF).