If you're using a DSLR, then the first thing you need to do is check the mount. Every lens has a specific mount that is designed to work with a particular camera. For example, Canon lenses use the EF or EF-S mount, while Nikon lenses use the F-mount. Once you've confirmed that the lens mount is compatible with your camera, then you need to check the aperture. The aperture is measured in f-stops, and it indicates how much light the lens can let in. A lower f-stop number means that the lens has a wider aperture and can let in more light. Finally, you need to check the focal length of the lens. The focal length is measured in millimeters, and it indicates how zoomed in or out the image will be. A shorter focal length means that the image will be wider, while a longer focal length means that it will be more zoomed in.