Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word: You're holding onto a sports ball in your hand, let's say it's a baseball for this mnemonic's sake. You throw it at your friend who's going to catch it. As it flies through the air, though, it transforms into a tomahawk (たま). Imagine your shock and fear as you see this happening, flying towards your friend! It's going to stick in his forehead. He can't catch this with his glove!

Also notice that it has the same reading as the vocab word 玉. So, if you know that one, you may know this one too! They're kind of similar.

This can be read きゅう too, if you're talking about more of a globe sphere than the ball sphere you throw.