Excellent advice from Rick and I would add that location and selection of what shots to take are critical.
When you start with a somewhat noisy image from having to use a high ISO, you really don't want to crop at all. Your lens complicates this further because you lose a full stop going from the wide to the narrow end of its focal length. You would love to be close enough to get the image while still in the f4 part of its range but that probably won't happen however the closer you can get to the action, the better.
A lot of sports venues, particularly those catering to younger/less profitable events, have very uneven lighting across the playing area so try to choose the best illuminated area and concentrate on it.
The sport determines the general minimum shutter speed you can use to avoid player motion blur and for most sports 1/640 is the general safe minimum with higher being desirable. But that shutter speed coupled with poor illumination with your camera and lens setup is likely to result in unacceptably high ISO. Because of this, you may want to focus your attention on those points in the action where a player momentarily freezes so that action motion blur won't be an issue and then you can potentially get down to the 1/250 speed Rick mentioned although you will need to hold the camera VERY STEADY because you are now getting into the shutter speed range were difficulties with camera shake will be an issue at the mid and beyond points of the focal length of this lens.
If you can get close enough for an 85mm lens to be useful, the Canon EF 85 f1.8 could be a good low cost upgrade lens to consider and a good used copy shouldn't be too expensive. I used the EF 85 f1.8 fairly often for indoor soccer and basketball in really badly illuminated venues. It provides very good images wide open and provides very rapid focus acquisition; I think it and the EF 400 f5.6 were Canon's two best "value" lenses for sports with both performing far better than they should given the cost relative to the more expensive lenses.
You should also shoot images in RAW instead of JPG because the processing software can do a much better job of noise reduction with RAW files; you have access to the free Canon DPP software with your camera and I use it for the majority of editing. Shooting in RAW also means you don't have to worry about setting white balance for image capture because you can dial that in during post-event processing.
My shooting setup for sports is always shoot in RAW with manual exposure; set aperture to widest available setting and shutter speed as fast as you can without ISO becoming unacceptably high and set ISO to auto to complete the exposure triangle. Set AF to servo mode and I use a single AF point for most sports. Using a single point puts more load on the photographer to do the job properly but the trade-off is even more important with less advanced cameras like your 600D because it doesn't have the extremely fast AF system of more advanced cameras and having it choose where to focus from an array of points further slows AF performance.
The bottom line is you need to concentrate on the types of images you can capture with the setup you have and that means in a poorly illuminated venue you won't be capturing sharp fast action shots because you won't be able to set the shutter speed fast enough. I use Canon 1DX III bodies for sports and these have incredible low light performance with their full frame sensor and I couple these bodies with f2 and f2.8 lenses but even with fast glass I end often end up with capture ISO at 6,400 and above which isn't an issue with the 1DX series but that would be unacceptably noisy with a 600D. So you need to drop shutter speed as needed to keep your ISO within reason and that will restrict the types of action you can successfully capture within an event.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
