Your built in flash probably isn’t powerful enough to do you any good unless you can get very close (10-15′). If you’re going to use flash you’ll need a more powerful external one.
Absent that, try turning the ISO setting up (as high as it will go if the results at that speed are acceptible) and use shutter priority with a speed high enough to stop the action – at least 1/200 or so.
A faster lens would definitely help too. I think Canon makes a cheap 50mm 1.8 prime. If you’re using an 18-55 right now and don’t feel you need something longer that might work.