Hi Tim,
Looking at your profile, it seems to me you're only rescue diver with PADI?
If this is correct, you have to do the Divemaster course first. The DM course can be done within about 8 -10 days. But to to the Instructor course straight afterwards - trust me, that would be a loooooong 3 weeks!! Ideally, I would advise that potential Dive professionals Dive as much as they can before going down the instructor line. PADI require a minimum of 60 to complete Divemaster course and a minimum of 100 dives for Instructor course. In my eyes and many others on the board that's not even nearly enough!
Teaching Diving is a great way to earn a few extra $$$ part time, and take your mind off your primary studies, but it's also a very responsible part time activity. So don't rush into it. Get comfortable with SCUBA, and dive as often as possibe. What I might suggest is take this summer to do the Divemaster course. Take time at it, and enjoy it. As I mentioned, it's curriculum can be scheduled around 8-10 days, but taking a month or two and doing an "internship" is of much more benefit. Then spend a year or two working with students as a Divemaster, learning what it's like to be role model in diving. You'll also get a chance to observe what instructors do, what kind of things they do, what different ways do they teach.
Hope this helps,
Best of luck,
Scubafreak