While the Internet helps us spread the word and disseminate basic information about the sport, the best way to learn is to be taught by other people. There is no substitute for a good teacher. One good way to start is to get the instructional video, "Tricks of the Trade", by the master himself, Kenny Shults. It's available at a reasonable price from the World Footbag Association. (Note: it's available for both the North American TV system and the European (PAL) system -- if you live outside the U.S., be sure to specify that you want the PAL version.)
Another great way to learn is to get involved in the local organized footbag scene (if there is one), and/or to go to a footbag event (such as a tournament or festival). There is an on-line footbag event list at http://www.footbag.org/events, an on-line list of footbag clubs around the world at http://www.footbag.org/clubs.
See also, How Do I Learn To Play Footbag?.