To start out with juggling you need some form of instruction. Most people have the wrong ideas about how to juggle, formed from cartoons and other media. One great place to learn how to juggle is on YouTube. http://www.youtube.com/watch?v=kCt1bmSASCI is one of the videos I used to learn. Once you think your good you can try to do some tricks, http://www.jugglingtricksunlimited.com/Tricks.asp?ID=400 . This site has hundreds of links and tutorials for tricks using three balls and more.
But it doesn't end there! When you have mastered three balls you can step it up and move on to four balls. Eventually five, six or seven! But the point isn't to be the best, it's to have fun.