What do you put in a sippy cup?

This is a really dumb question but my son is now 6 months and he can hold his own bottles now so I bought him a transition sippy cup since i read online to start them on one around 6 months but my question is what do i put in it? His formula? Water? Juice?

So far he’s only drank a small amount of apple juice to help with constipation once. And he’s never had plain water.

Any suggestions? I’m a first time mom and a little confused 😂