If you are a lefty, 7% are, you should use your mouse with your left hand. Windows has tools that will help you. The setting will switch the two buttons. The default setting (for right-handers) is that the left button (under your index finger) is the primary one and is used for most actions. The right is used for selecting a menu for the item selected.

In Windows 7, the globe on the lower left, Control Panel, Hardware and Sound, Mouse. If you have switched the buttons in Windows, you will need to switch the words right and left as you read. If you are a lefty, you are already used to this. The correct setting is to have the primary button under your index finger whether you are right or left-handed.