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$.
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.