here's my advice, when you pick a new lens for your camera, the first thing you should remember is the brand name of your SLR, lenses are not interchangeable! so if your SLR is Canon, make sure you look for Canon lenses only 
Once you have the brand, you would have of about 30 different lenses to choose from and this is how you can narrow down your choice:
1. Decide on the focal length that you want for your camera
2. Determine if you want a zoom or a prime lens for your camera
Prime Lens is a lens with only one focal lenght. It comes in different focal lengths ranging from wide angles to the longer telephoto angles.
Zoom Lens comes in a range of focal lengths in one lens. this is actually a much more convenient choice as it gives you flexibility to choose both wide and long focal lengths without having a switch lenses in the middle of the shoot
3. Select your desired maximum aperture
4. choose either a first or a third party lens
5. compare the extra feature each lens has to offer and how it can help you
6 ask advice or read reviews
After this, this would have narrowed down your choices from 2-3 and the only thing you have to worry is budget and which of the extra features will be more beneficial for you