Firstly, the most important and simplest thing to remember it is always best to format the memory card in the device you are going to use it in. (A new memory card, straight out of the manufacturer's packaging generally should not require formatting). For example, if you have a Canon EOS 600D and a SanDisk Extreme SDHC Card, this memory card should be formatted in the camera.

However, you may have taken photos and have copied them using a memory card reader to your PC or MAC for archiving. When you have copied and backed up your photos safely, you may then wish to format the card, clearing off the data and freeing up space.  It is always good practice to format memory cards after each photo shoot, when all the data has been copied and backed up.
 
Follow these simple steps to format your camera memory card quickly and correctly with the minimum of fuss:

1) Insert the memory card into the a suitable memory card reader connected to your computer
2) Click on the "Start menu" icon in the bottom left corner of the Windows desktop screen
3) Double click "my computer" on the right side of the dialog box that pops up
4) Find the drive letter designation for the memory card listed under the "my computer" heading on the left side. Right click on the drive. A dialog box will open. Click on "Format"
 5) Select the type of formatting under "File system". Choose the file format or FAT32 for the formatting type for any card over 4 GB**
6) Select either quick format or regular format by checking or unchecking the "quick format" box at the bottom of the dialog box.
7) Once the settings are correct click the "Start" button.

This process normally takes 5-15 seconds, however the time can vary depending on the capacity of the card - so please be patient!

IMPORTANT NOTES TO CONSIDER:
**Please note formatting a memory card is an irreversible function, so it is important you have a copy of all the data you require from the card first before you begin the process.
**Memory cards of 2gb capacity and under should only be formatted in FAT and not FAT32
 **If you do not format each time you have copied your images to your computer, there is a possibility that your new images could become corrupted. This may happen because the card is using the old image index file which still has listings for files that are no longer on the card if you deleted them instead of formatting the card.