What should I juggle?
If you're only going to be doing a simple 3-object cascade, you can probably juggle almost anything with practice. Juggling bowling clubs, tennis rackets, and even basketballs is quite possible with such a relatively uncomplicated pattern. But if you want to learn more complicated things, you are definitely going to want to have good juggling balls that make things easy on you.
It might not seem all that important now, but in the future it will be very much simpler if your juggling balls are all the same. Although it's best to have balls that don't bounce or roll around, most small sports balls (tennis ball size) should be OK, and bouncy balls that are big enough are great too. Hacky sacks and beanbags are even better because they are very manageable.
The best solution - but the most expensive - is to go order from a juggling retailer. They generally have a pretty large selection of beanbags and balls designed specifically for juggling, although each ball can cost anywhere from $3 to $30. Check out Dubé or Renegade Juggling, both of which have a great selection. Remember not to order anything nonspherical from there unless you're either teaching a class or learning
A less-expensive alternative is to make your own beanbags. This actually isn't very difficult; I've made a bunch of them, and although they haven't turned out perfectly they are getting better. I would suggest going to IJDB's beanbag creation guide to learn how to make them well. Marylis Ramos's Sewing Patterns for Jugglers is an excellent collection of patterns that you can use to make your own juggling balls. Just make sure to select the right fabric! Remember, a weight of about 100g is good for 3-ball juggling, but if you want to do 7 or more (or even 5) you will want to consider a mass of about 50g.