Canon cameras have a focal length multiplier of 1.6. With a 300mm lens attached to your camera it will give you an equivalent focal length of 480mm at the long end, which should be good for photographing your daughter playing basketball. You will no doubt have to adjust white balance for the lighting, and raise the ISO to get a faster shutter speed if you want to avoid camera shake and capture the action.
Remember, too, the more you zoom in the more light you'll need, so you might be better off not zooming in fully and then crop later when editing.
The Fred Miranda site is an excellent place to learn about Tamron (and other lenses):
http://www.fredmiranda.com/reviews/index.php?cat=48It's going to be difficult to find a faster lens at that price range.
Do know, however, that as a general rule third party lenses autofocus very well in good light but struggle more than Canon lenses in low light. Hopefully, you'll have enough light, otherwise you could consider manually focusing.
Enjoy your new camera. Take lots of test shots and give yourself some time with both the camera and the lens.
Related reading:
Focal length multiplier:http://www.digicamhelp.com/camera-logs/dslr-dilemma/focal-length-multiplier/You may find our Guide to DSLR lenses useful:http://www.digicamhelp.com/accessories/dslr-accessories/lens-uses/
