You would be astonished at how few students just stop by to chat about a subject because they're interested in it, and at how memorable a faculty member finds this. However, this is a professional courtesy rather than a student right. Early in your college career, make an appointment to discuss your plans for graduate school with your advisor or other faculty member. Do this as soon as you develop any interest in grad school. This might be as early as your sophomore year.
Faculty members can offer much good advice on the general graduate application process.
During fall of your senior year, make an appointment to request letters of recommendation with faculty members. Discuss your plans with them. Give faculty members as much time as possible to write the letters. A month or longer is a good amount of time. Most of the faculty member's effort goes into writing the text of the first letter.