To get a really nice wide lens for landscape I would suggest the Canon EF-S 10-22mm f/3.5-4.5 USM. You are using a body with a crop sensor, so you need to consider that your actual lens focal length would be mm x 1.6 so at 10mm its the equivalent to 16mm on a full frame sensor, or 35mm camera on film.
I have the 10-22 and find it nice and sharp and has a solid construction.
If you want a more versatile lens I would say consider the 17-55mm f/2.8 IS USM, also a great lens and this will give you a more versitile range if you intend to shoot a few portraits.
One thing to keep in mind with both these examples is they are EF-S lenses, which means if you upgrade your body to an EF type (full-frame) then these lenses are not compatible.
If that worries you Have a look at the 16-35mm f/2.8 L series lens
as suggested a nifty 50 is never a bad addition to your camera bag, the f/1.8 version is cheap as chips and takes brilliant images for the cost