You want a long exposure with a small aperture. Lightning is really bright and will flood the camera with light for short exposures. Plus having the exposure as long as possible maximises your chance of getting a bolt in frame.
If you're going to try and capture the lightning by hand you should:
i) Set the camera to manual focus.
ii) Set the camera to manual exposure and set the exposure to be underexposed by a little (like half or 1 stop).
iii) Get lucky.
Unless the storm makes the sky completely black which allows you to use multiple-second long exposures, you'll find it really hard to get lightning shots during the day, unless you use either something to automatically trigger the camera.
I've used <a href="http://chdk.wikia.com/">CHDK</a> firmware on my little point and shoot canon.
But for a 500D you can try the <a href="http://magiclantern.wikia.com/wiki/Unified">Magic Lantern</a> which has a motion detection mode. The motion detection also gets triggered by lightning flashes.