The focusing ring on the lens. I'm not familiar with Nikon so I don't know what you're saying. The camera is in automatic and it focuses for you, but when you use a manual setting it doesn't focus? There should be a switch on your lens that says AF/MF. Again im not familiar with Nikon or what lens you have (kit lens, zoom lens, fixed lens). But in most cases you move the switch to AF to let the camera focus for you, move it to MF and use the ring on the lens to focus on your subject until it's crisp in the viewfinder.
Ben