OK, thanks for posting back your info. In order to format the hard drive you have to boot your Mac with the Snow Leopard disk. Once it boots to the disk, accept the language and the license. Go to the top menu and select Utilities, then Disk Utility. From there you should be able to find your way around to do the erase and reformat.
I should mention that we do not normally recommend erasing and reformatting your hard drive because of slow downs. Slow downs can be caused by many things: not enough memory, hard drive space lacking, hard drive fragmentation, and others.
I suggest while you're booted from the disk, to run a verify and repair on the hard drive before erasing and formatting. If your hard drive has errors or is dying, that could cause slow downs which will not be cured by erasing and reformatting.
And don't forget to make a backup before doing any work on the hard drive. And keep in mind that erasing and formatting destroys all data.