Nikon lenses (and almost all other DSLR lenses) have the same minimum focus distance across their whole zoom range. This means that to get the biggest magnification for close-ups you need to zoom in to the maximum setting. This is where you find out your humble kit lens can often give you better close-ups than you thought.
Note that this doesn’t work with compact cameras. With these, the zoom and focus mechanisms are linked in a more complex way, and zooming in means they can’t focus as close, so you don’t get the same benefit.