Hmm...the S500 is a totall point and shoot camera so you can't set the aperture or shutterspeed.
Set the camera for -1 exposure compensation and use the internal flash for anything closer than 1 foot.
For an external flash, you'll be wanting something cheap. Check out something called the "Nano Flash" which is sold here:
http://www.fantasea.com/product_cat_...p?cat=5&sub=15
It's a little quantum flash in a clear housing. This flash can be set to ignore 1, 2, 3, or 4 preflashes and it works great w your camera.
Cheers
James