Scotty provides great advice and if you can't swing the cost of a 135 F2 right now if you can get close enough to the action the very inexpensive Canon 85MM F1.8 can work decently. It has OK sharpness even wide open although that improves when it is stepped down slightly. I am confident it will focus at least as fast as your current zoom and it should be significantly faster focusing in low light with its much wider aperture. Before I upgraded equipment it was the lens that worked best with my old 1D Mark II body for horribly lit indoor soccer. With an 85mm lens, even on a crop body, you need to get close to the action but it will provide very usable images in lighting conditions that are completely beyond the capability of your slower zoom lens on that body.
When upgrading lenses do so for the future because a good lens will be useful long after the body itself has become technologically obsolete. If there is any chance you will go with a full frame body in the future then avoid EF-S lenses which only work with a crop body like your current Rebel. If Canon makes the lens length and aperture that you need then my preference is to go with Canon over the third party suppliers.
You will likely have the most success setting your Rebel to choose aperture priority (Av mode). Manually set your ISO to the highest that provides a usable image (probably 3,200 but maybe 6,400 will work with your camera body), set the lens wide open (its lowest number F stop), and then the camera will choose the shutter speed necessary to provide a proper exposure. You could also try full manual mode also setting your shutter speed manually in addition to the aperture. 1/500 will freeze a lot of soccer and football action but certainly not all plays (like a player rapidly approaching your position) and 1/250 can catch some nice images when things are temporarily frozen (quarterback dropped into the pocked and stationary while waiting for the receiver to get in position, a soccer player who has paused dribbling before switching direction). But until you gain more experience I think that ISO to maximum usable and lens wide open with the camera choosing shutter speed will provide the most "keepers".
Experiment some before the next game choosing a low light setting to see how high you can push the ISO without losing too much image quality. If you get there before the game you may find another photographer who has a bit more experience willing to help you out. For football games I will get there in time for the team's pre-game warm-up and to talk with the coaches before the game starts and during that time I will take a few shots but then once the team leaves the field there is downtime until the game is ready to start. I have helped several parents with setting up their camera along with some quick tips on capturing images of "their" players. With your current setup you aren't going to be able to capture fast action shots in low light but there are a lot of good images to be captured including post-play celebration, interactions between coaches and players, player expressions and interactions on the sidelines, etc.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
