Go to an empty parking lot or school yard playground. Plan on being there at 6am every morning. Practice riding until 7am every morning until you get good enough to ride in the scarcely populated suburbs.
After about a week or two of practice in the playground, riding everyday for an hour in suburban streets will teach you how to ride in the big city.
Get a book on the rules of driving automobiles, so that you will know what various traffic signs mean and what drivers may legally do when driving. Stay in the bicycle lanes whenever possible. Always yield to pedestrians, even when they're wrong, if they're in the street or on the sidewalk (you shouldn't be on the sidewalk).
Good Luck!
SlimRider
PS.
When you do buy a bike, make certain that it's from a bike shop near your school. That way, you can go to them directly, if there's a mechanical problem and you won't have to ship it or pay exorbitant repair costs.