In order for us not to be too technical, you need to give more info, such as why you need to reformat.This is not a usually needed operation.
But, if you must, double backup all your data. Insert the Tiger dvd. restart holding down the "C" key to boot from the dvd. There should be the option of launching DiskUtility. This will give you the opportunity to erase or partition your disk. Click on the button that asks you whether it is OK to erase all your data, (YOU HAVE BACKED UP!!). Click OK and your disk will be reformatted.