Can someone please give me an example script of how I would use this on, say, a tree?

I know i need to give the tree a reference name and make it a persistent reference, but how would I use getcrosshairref to see if the player activated it?

You don't have to activate the object. It's enough that the crosshair is over it within the activation distance.

ref rTreeREFset rTreeREF to GetCrossHariREF

Thanks, but then what would I put if I wanted to give the player ten caps when he activates it?

Why not put a script with an OnActivate block on the tree?
Otherwise, you'd have to keep looking at GetCrossHairRef until it was your tree's specific reference, then use IsControlPressed to check when the player pressed use.