Make sure to copy anything you need before formatting, which erases everything on whatever media you format.
Step2
Single-click the disk's icon on your desktop to format.
Step3
Pull down the “Special” menu from the top of the screen and select “Erase Disk...”
Step4
Enter whatever name you want in the “Name:” field in the window that pops up. If the disk is a hard drive, select “Mac OS Extended” from the “Format:” pulldown. If the disk is not a hard drive (such as a Zip Disk or other removable media), select “Mac OS Standard” for the format.
Step5
Press “Erase.” The drive will be formatted and ready to use.
Tips & Warnings
You can find more advanced articles on formatting right here on eHow.