I'm actually having a base kit, it's a Canon Rebel T2i with a Canon 18-200 lens and a Sigma 70-200 2.8 lens. Can you recommend some tutorials, we have something at school but it's not really getting deep into the use of the lens.
The Sigma actually trumps the Canon here. Although an entirely different focal range, it is pinsharp even wide open at f2.8 - cut yourself at f4.0 where the Canon needs a bit of work and really doesn't get to where the Sigma sits anywhere through its range.
Dpreview says about the Canon:
Huge 11x focal length range, ideal general purpose and travel lens
Excellent image stabilization system, consistently delivering three to four stops benefit
Good central sharpness at all focal lengths (but inconsistent corners)
Attractive rendition of out-of-focus regions of the image
Improved build quality in comparison to the 18-55mm and 55-250mm kit lenses
Well-placed zoom lock switch
Poor sharpness across much of the frame at 18mm and wider apertures
Pronounced barrel distortion at wideangle, and pincushion distortion around 50mm
High levels of chromatic aberration at either end of the zoom range
My advice: use the Canon as walk-around standard lens, take the Sigma when you won't run into scenes that need wideangle or extreme sharpness. The Sigma also has wonderful depth of field at f2.8 - lift your subject right out of the background.