Well basically the only modes on your camera which you really need to be concerned about are;
M - Manual
Av - Aperture Priority
Tv - Shutter Priority
Now each of those you will genuinely use depending on what you are photographing, most of the time when I'm just taking candids and snapshots I leave it in aperture priority, that will sort out the shutter speed for me whilst I control the aperture through the wheel, ranging from the lowest number (the lens open widest) which will give me a more shallow depth of field (giving me a nice blur behind and infront of the subject I am focussed on) but at the same time also let a lot more light in, where as a higher number will restrict the amount of light entering through the lens but in return this gives me a much greater depth of field - this is very useful for landscapes.
Also whilst you're in aperture priority you have easy access to changing the exposure level from -2 ev up to +1ev, where 0 is according to the camera.. exposed just right - however that won't always give you the effect you desire so sometimes you may need to change that. That can be done by holding down the "Av" button and then scrolling the wheel on your camera whereas if you were to scroll the wheel on your camera whilst in Aperture Priority mode without holding this button down all you will end up doing is changing the aperture setting.
Manual setting I tend to use when I want a lot more control over my exposure, either because I know exactly what the lighting is like and how I want to control it, e.g. on photoshoots under studio lights or with flash units.
Shutter priority I tend not to touch too often as I figure if I'm going to be changing the shutter speed manually then I may as well stick it into Manual, I don't like the camera telling me what aperture setting I should be using. So that could be setting your shutter speed very high such as 1/4000 if you want to capture very fast objects, or very slow with speeds greater than 1 second if you want long exposure shots to capture light trails or blur people as they're walking around a fixed backdrop etc.
But like I said earlier, I'd pretty much just ignore all of the other shooting modes on the dial of your camera, if you just want to do some point and shoot for now then I think just leaving it in Aperture Priority (aV) should do fine for now.
There's also the focussing, you can either leave that on auto and let the camera do the work or do it manually. Canon cameras have a very sophisticated autofocussing system, which I don't quite have time to explain through at the minute! But there is a very good video seminar flying around the internet somewhere clearly explaining how it works. It's quite long but well worth watching if you really want to understand how the camera works. I'm sure someone can point you into the right direction on that as well.
I just remember I forgot to mention the ISO.. That can be accessed through the button between the on/off switch and the dial. In a nutshell, basically the lowest setting meaning you get less light but a clearer image whereas a higher setting means more light will come in but the image will be grainier.
There are a lot of very good tutorials in the section of this site, some of them not so specific to the 500D but once you've got a firm grasp of how the basics of SLR cameras work then it's quite easy to apply that to whatever camera you're using.
If there's anything else then I'll gladly respond to this post when I've got the time and I'm sure all the other members of TP will be more than happy to help out! Some of what I said may be frowned on by other members (such as my hatred for shutter priority!), but a lot of how you use the camera is very much down to how you best feel comfortable doing it.
Good luck playing with your camera.
