Ahhhh, such grace, such dexterity (ooh! that's a BIG word!).
He's obviously been practising...

Now, the important thing here, is to THROW with one hand, and catch with the other
See if you can do it, got a ball?

When you have the hang of it, the next most important thing, is to try to make sure the ball goes just as high, when you throw from the left hand, as from the right... got it?

This takes a while to get the hang of...

Jugglers rule number ONE: When you can do it twenty times without stopping, you've got the hang of it

Try to stick with this until you can throw the ball up in the air twenty times, without dropping it. Once you've done this, you're ready for the next big step:
Juggling TWO BALLS!