How do I monitor a new track while one is playing?

Hey
Im just using my MacBook plugged into a tiny mixer which plugs into speakers. How would I monitor a track that I want to mix in, in a way that doesn't require it to play through the speakers? Seems like i need some kind of hardware I don't have... Thanks!
Im just using my MacBook plugged into a tiny mixer which plugs into speakers. How would I monitor a track that I want to mix in, in a way that doesn't require it to play through the speakers? Seems like i need some kind of hardware I don't have... Thanks!