The first step is to set the camera and/or lens to manual focus. For most cameras, this is just a switch on the lens, however on others it may be a switch on the camera body or an item in the menu.
The switch is almost always an MF/AF switch, you need to set yours to the MF position.
If you’re not sure how to do this on your camera, refer to your camera’s manual.
Find the focus ring
Next, find the focusing ring on the lens. If you’re using a prime (one focal length) then there will probably only be one ring. If you’re using a zoom lens, it may be that the focus ring is in fact the front of the lens, or the thinner of the two rings. If you have two rings on the lens, the focus ring is the one which doesn’t zoom the lens.