hey people what lens should i use for a 400D
i want to shoot images of people like they will focus only on the person but the background would be blurr
One single lens to do all 3 that you have mentioned ? Not available, regardless of maufacturer. For sports, you need telephoto. For portrait - focal length 50 mm - 100 mm ( some prefer slightly higher, don't be surprised professionals use even longer focal lenses )
For landscape - use wide angle. How to categorise the different focal lengths in different sections, hope this helps : http://www.canon.com.sg/index.cfm?fuseaction=digitalcamera&prod_type=lenses