Take the MCAT by June of your senior year. These are the dates when it is offered:
http://www.aamc.org/students/mcat/2007calendar.pdf. Start reviewing about four to five months in advance. Light review before then couldn't hurt, but you don't want to get burnt out. Make sure to include practice exams in your study schedule. Also have already completed and largely understood the pre-reqs diverdoc mentioned.
Use the score you'll get back one month later and your GPA to figure out a range of schools to apply to the summer before your senior year. You want your primary AMCAS application in as early as possible, sometime in June or July that summer. Interviews and secondary applications will start soon after. Hopefully around March of your senior year, you'll have an acceptance in hand. Good luck!