I assume that you're using the EF 70-300mm lens, which is a lens that needs lots of light, as does your camera. For shooting sports, especially in low light, almost requires a lens with an f/stop [maximum aperture] of f/2.8, or faster.
If you set the camera to shoot RAW, instead of JPEGs, you can use the Canon DPP software to correct somewhat for dark images. Unfortunately, there is nothing you can do about blurry images. You can also crop images down to size, too. The DPP software also allows you to do some noise reduction in the images, too, which can allow you to shoot at higher ISO values.
http://www.learn.usa.canon.com/resources/articles/2016/dont-be-afraid-of-the-dark.shtml
I would strive to shoot at the shorter end of the zoom scale with the 70-300mm. The maximum aperture on your lens narrows [reduces the light] as you increase the focal length. Remember, you can always crop images in post processing, to simulate a long zoom.
The best shots are a result of getting yourself positioned for the shot, not from having a "long" focal length lens. Doing so takes a knowledge and familiarity of the sport. Try to attend a practice session, because you probably need it, too, with the camera.
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