Make sure the drive is connected to your computer (internal drive should be, external drive needs to be plugged in.) Then click Start > Computer and find the drive in the list of drives. Ensure any data on the drive that you wish to keep has been copied to another drive, as formatting will remove all data. Right-click on the drive's icon and select the "Format..." option to bring up the Format window.
Select the required file system (you'll probably want to stay with NTFS) and give the drive a new name (called a "Volume label"). If you just want to erase a drive that has already been formatted, consider using the "Quick Format" option; otherwise leave this option unchecked. Click "Start" to begin the formatting process. A full format could take several hours.