If your hard drive is an external drive, connect it to the computer. Launch Disk Utility from the Applications:Utilities folder.
From the list on the left select the drive you would like to format
Click to select the Erase tab.
From the Volume Format dropdown select the format you would like the drive to be in.
Enter the title for your volume in the Name field.
Press the Security Options... button to specify how to erase the drive. You can choose: Don't Erase Data, Zero Out Data, 7-Pass Erase, or 35-Pass Erase. Click the OK button to continue.
You are now ready to begin the format. Click the Erase... button to start.
From the popup that appears click the Erase button to confirm you would like to format.
You will notice a status bar appear with the progress of your format.
Once the format has completed you will notice the disk remount in Disk Utility!