We have a whole tutorials section on the main page, under "Learn." Check out the roll tutorial here:
http://www.americanparkour.com/content/view/264/377/ and here's a video:
http://www.youtube.com/watch?v=B9Jdx1cHKHUI'd suggest starting on grass without any sort of drop. Start from a crouched position and work on it. Once you're comfortable, stand up a little bit and go from a slightly more raised crouch position, then move up again and again until you're standing.
At some point, try taking a few steps and then going into the roll. Once you're comfortable with that, take a few steps, take a little hop (to simulate a drop), and then go into the roll.
(This progression should be spaced out over weeks, if not months).
From there, move onto rolling on different surfaces, rolling from SMALL drops (start with 4 inches, then 8, then 12, etc).
Slow progression is what it's all about.
