The default camera app does not have a zoom feature, but you shouldn't want one anyways. Cell phone cameras (at least most of them) do not have optical zoom lenses, which means all the zooming is just a digital manipulation of the the image. A good digital zoom relies on very advanced algorithms and a lot of processing power to get a decent outcome, which just isn't possible in real time. It leaves you with a much lower quality photo.
The best way to zoom is actually to crop the photo after taking it. This way none of the detail is lost. If you absolutely must enlarge it, you're better off using something like Photoshop which will actually take the time to process the image thoroughly, though it's still not a perfect enlargement.