Other Tutorials:

Juggling Poet (text-based)
JugglingStein (video)

     The Fountain is one of the most basic four ball patterns, and almost always the first one that jugglers learn. The trick itself is simply a pair of Two-in-ones being juggled asynchronously (this pattern is technically called the Asynchronous Fountain). Unlike many three ball tricks, the Fountain is generally not a pattern that can be simply "figured out," and instead often requires a few weeks of practice to get mastered. Before you start learning the trick, it is important for you to be able to juggle a Two-in-One with each hand, and it helps to know the Half-Box (441).

    The first step in juggling the Fountain is to practice with three balls. Start with two balls in your dominant hand, and one in your other. Throw one ball from your dominant hand as if in a Two-in-one, and the toss the ball in your other hand in a similar fashion. Once the first ball has almost returned to your dominant hand, throw the last ball up in the air, and then catch them all.
    Practice this step extensively with both your dominant and non-dominant hand leading. Remember, you will need to be able to maintain a Two-in-one in each hand, so always practice with both hands equally. The next step to learning the Fountain is a  four ball version of the step above, with each ball being thrown once. Start with two balls in each hand. Then, throw one ball from your dominant hand, followed by one from your non-dominant hand, followed by the other from your dominant hand, followed by the other ball in your non-dominant hand. Catch all the balls after these four throws.
    Once again, practice this extensively. Make sure to do this exerice starting with each hand. After you can do this step cleanly and consistently, it is only a matter of increasing the number of throws, and you will be juggling the Fountain. I would recommend adding just one or two throws at a time for the first few cycles, then increasing the rate as you become more proficient. The Fountain is the backbone of many more advanced four ball tricks, so it would be wise to invest a lot of time into mastering this pattern.