Usually, the first lens selection question to ask is: "What focal length(s) do I need?"
Most people don't buy a lens simply to try it out, but instead have a real purpose in mind for their purchase. The angle of view or angle of view range provided by a lens establishes the distance the intended subject should be photographed from to get the desired framing. That distance drives the perspective realized in the image. One does not need to be far from a person to include their entire body in a wide-angle lens's angle of view, but a telephoto lens requires significantly more working distance and delivers a different perspective and look.
Generalizing, select a wide focal length (lower mm number):
Again generalizing, select a telephoto focal length (higher mm number):
While the above lists push toward higher and lower focal lengths, photographers typically use the middle focal lengths the most, and the full-frame 24-70mm range is about perfect for general-purpose use. If possible, mount an existing (owned, borrowed, rented) lens to the camera to evaluate the angle of view provided, helping to determine your need. Even if that lens does not have all of the features you need, seeing the angle of view firsthand will allow evaluation of how your subject will appear at specific focal lengths.
Another good way to determine the right focal length(s) is to review sample photos. Find images like you wish to capture, and then find out what focal length was used to make the shot. Our galleries include the focal length setting used for each picture. Most of this site's reviews have focal length comparisons included in them, another way to understand what needs are met by specific focal lengths.
A note: You must keep the FOVCF (Field of View Crop Factor) of specific Canon DSLRs factored into your expectations. APS-C/1.6x FOVCF bodies require wider focal lengths than full-frame bodies for the same perspective and framing.
I will make specific focal length suggestions in my lens recommendations.