ATiTool is an overclocking utility which allows you to overclock your VGA card.
As for OmegaDrivers, it's an alternative driver apart from ATi's official Catalyst driver, which some people reported to say gives higher overclocks or better performance.

As for getting started, there's nothing to get started or to worry with.
JUST DO IT!
Well, slowly overclock with small increments.. maybe 5 to 10mhz.. (which I don't usually do.. )

Then try a 3dmark run test... Then if you see no artifacts on the 3dmark screens, then continue on..
Or you can use ATItool's scan for artifacts option to scan for artifacts.

All the best and have fun burning your GC~!