As with buying a camera there are a lot of things to consider, so this question really can’t be asked without asking many more questions first. Here are some of them:

What exactly are you going to be photographing?
Your photographic goals and objectives essentially inform all of the other decisions. Will you being shooting landscapes? Portraits? Your kids or pets? Flowers? Wild birds? Sports? Architecture? Will you be doing travel photography? Will you be hiking with your gear? These sorts of questions are critical to ask yourself first.

How much money do you want to spend?
You can spend huge amounts of money on lenses, so determining your budget, as with cameras, is pretty important.

Do you want to buy a used or a new lens?
The same questions apply as when purchasing a new or used camera.

Do you want a prime (fixed focal length) or a zoom (variable focal length) lens?
Prime lenses generally afford higher optical quality than zooms, except in the case of really expensive pro zoom lenses. However prime lenses mean you have to walk around more - you can’t simply adjust the zoom setting to get the framing you need.

What focal length or focal length range do you want to cover?
I’d sit down with your photos and think about what focal lengths you tend to use most and where there are gaps. Maybe you want an extreme wide angle, for example. If so there’s no point getting another telephoto.

Do you need a fast lens?
Do you want to do low-light photography without a tripod or flash? Do you want to do portrait photos while blurring the background? These applications call for a faster lens which can let in more light.

Do you want to buy Canon-built or third party lenses?
Third party makers build many great lenses, but many are also really poor - you have to do some research. And others have compatibility problems with existing cameras or may have future compatibility problems. Are the price savings worth it for the specific lens you’re interested in?

Do you care about the user interface and build quality?
A lens may be slow to focus or inconvenient to use but offers higher optical quality than another lens with a faster motor and a better designed UI. Ring USM motors are fast, silent and offer full-time manual override, but lenses containing such motors tend to cost more than slower, noisier non-FTM arc-form (AFD) motors.

What aspect of optical quality is important to you?
Obviously sharpness and contrast are pretty important to most people, but what about distortion? Many consumer zooms have a lot of distortion, making them unsuitable for architectural photography. They also tend to be more vulnerable to flare (lowering contrast or resulting in bright spots on the picture if a bright light like the sun is in or near the frame) and tend to have very slow maximum apertures.