Welcome to the Apple communities.
Please find a moment to tell us more about your machine, the model identifier, the OS version, and the total amount of RAM you have. It also helps to know where you are, nothing specific, just the country.
Then fill those same details in your profile for future use.
All of this needs to be shown for us to answer questions otherwise we are just working blind. Remember, different OS's and devices often require different answers for the same question.
If you are on Snow Leopard 10.6.8 or earlier, here is an Apple document that will help.