The big secret about juggling is this: it's easy. All it takes is a little practice and dedication. Anyone (yes, anyone) can learn, and most people become comfortable with the basic three-ball cascade in a matter of a week or two. You don't have to be an athlete. You don't have to be particularly coordinated. Everyday objects may slip from your grasp on a regular basis. Perhaps you walk in to doors more often than you'd like to admit. But you can still learn to juggle.
Lots of people learned to juggle from Juggling for the Complete Klutz. It's well-written, funny, comes with 3 beanbags, and lists for about fifteen dollars.
This online juggling tutorial is pretty good, and includes tips on making practice balls out of old socks.
Eric Bagai has a nice instruction sheet on how to juggle, and how to teach someone to juggle, which includes tips for wheelchair-users and for people with one hand or with hands that don't grasp well.