A few ideas/pointers to consider: (check your manual for specific info on selecting/setting the settings).
1) Focusing - best way to focus for action based photography is to set the camera to "Ai-servo" focusing mode. This means that so long as the AF is engaged the focus will continually re-check itself to make sure that its still in focus - good for when a subject (or yourself) are moving. Avoid Ai-focus as this is a mode that attempts to shift between Ai-servo and single shot AF depending on what the camera detects - and most of the time it gets it wrong.
2) Backbutton AF control - you'll have to check the manual for this, but setting the backbutton AF control is a very powerful feature. What this does is shift AF activation from a halfpress of the shutter button to a full press of the * button on the back of the camera (higher end camera bodies have a dedicated AF button on the back for this). This means that you can press the shutter button and it won't engage the AF at all; instead you have to hold the * button to get AF to engage.
This is great for allowing methods like the described "focus and recompose" method above and also for when you are in a situation where the AF isn't getting a reliable lock into the subject- allowing you to leave the AF off (ie not press the * button) and still take photos with the shutter button without the AF trying to shift the focus (combined with a USM focus motor which allows all the time manual focusing of the lens and you can shift between AF and manual focusing without having to find the switch on the lens body).
3) Set the AF points to a single point - typically on the rebel series bodies the middle point is your most accurate from the whole series. Using a single point lets you point the camera at what you want in focus, since the AF nearly always locks onto the closest contrast change in a scene that it can find - which is not always what your subject is.
4) AV mode - as said above get out of the auto modes and into the semi-auto - aperture priority mode is a great mode for working and learning in. In this mode you control the ISO and the aperture, whilst the camera balances the shutter speed to the meter reading and your set settings. Using a good wide (small f number) aperture to get the light in for a fast shutter speed is ideal, though on a lens like the 100-400mm being about one stop down from its widest setting (so f7.1 to f8) really makes a difference with getting a sharper photo.
Your ISO should be as high as needed and remember that if you use a higher ISO, but expose correctly you'll get less noise than if you underexpose a lower ISO. Typically I'd say ISO 400 should be usable - 800 a bit more tricky and any higher and you'll be losinga lot of quality on a rebel series body (however if you have to you have to - blur from motion is one thing you can't fix in editing). Note for a moving subject you want 1/400sec minimum (and with the 100-400mm at the 400mm mark you need at least 1/400mm to handhold and avoid your own shake affecting the photo as well).
