I don't recommend tennis balls for a beginner: they bounce everywhere, and you'll spend most of your time chasing the balls rather than juggling. Cheap juggling balls are around 10$.
Learning to juggle way back lasts century, I learned to juggle using lacrosse balls. Very bouncy and and a little on the heavy side. Standing over a bed helps if you are using bouncy things (they still can cause havoc in a bedroom when they bounce off each other though).
One of the IJA (International Jugglers' Association) videos that most impressed me is :
IJA Tricks of the Month by Zaila Avant-garde | Juggling Basketballs (https://www.youtube.com/watch?v=sH2E1m8Fseg). Not only does she manage the juggling but her parents let her do it indoors with all sorts of stuff around.
I actually learned contact juggling with a lacrosse ball, since its uniform color and texture make it hard to see the rotation of the ball. That way, you get the similar visual effect to a more expensive acrylic, but without the risk of chipping if you drop it while you’re still learning.
This. Something like a hacky sack also works very well. They don’t roll away from you, and they won’t drive your neighbors nuts especially if someone lives below you when they hit the ground.
