The first question I ask myself when preparing for a shoot is "What focal length(s) am I going to need?" This is also the first question you should ask yourself when you are selecting a lens to purchase.
Most people don't buy a lens simply to try it, but instead have a real purpose in mind for their purchase.
So, how large or small is your subject and how far away from it will you be? Subjects range in size from a tiny insect up to an entire city. Distances range from as far as you can see (the moon and planets) down to a few millimeters.
Keep in mind that perspective changes with the distance of the subject. Taking a picture of a person's face from a very close distance will yield a mountain of a nose (BTW - this is an interesting effect to try for fun). There is usually some limitation that prevents you from getting too far from your subject (such as a wall).
Wide angle lenses let you get more subject in your picture, or let you get closer to your subject. As mentioned above, distorted perspectives must be guarded against when using wide angles unless they are desired for effect.
Telephoto lenses typically compress the subjects in a picture, but can compress all of the depth features out of it as well. Much more working distance is typically needed for a long telephoto lens. I can't say I've ever heard the Canon EF 500mm f/4 IS Super Telephoto Lens recommended as a people lens (other than for sports action).
So, you must find the right focal length(s) for you. How do you determine what focal length(s) are right? Use an existing (owned, borrowed, rented) lens to determine your need. Even if it does not have all of the features you need, you can get an idea of how your subject will look at the specific focal lengths.
Another good way to determine the right focal length(s) is to review sample photos. Find ones like you wish to take and then find out what focal length was used to make the shot. Our galleries include the focal length setting used for each picture.
Our reviews often have focal length comparisons included in them. This is another good way to understand what your needs are.
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 some focal length suggestions in my subject-specific lens recommendations.