First practice throwing one ball from hand to hand. Try to keep a similar arch and height with each toss. Practice this then go to step 2.
Next add a second ball. Practice "One, Two, Catch." Throw one ball to the other hand and throw that hands ball just before catching the first. Practice catching with both your right and left hand.
Finally, add the third ball and practice "One, Two, Three, Catch." Try catching with both hands again. Keep practicing this and adding more tosses until you can keep it going.
Now you can juggle!