If you receive a letter from the IRS requesting more information, the first thing to do is read the notice thoroughly to find out why it is being sent. Your notice will explain the reason for the letter and will give you instructions on what to do.
Generally, the notice or letter will have a list of items that they need from you or your spouse (if applicable) and they will provide a mailing address or fax number to get this information back to them so they can finish processing your return.
It is important to respond within the time-frame given so that your return can be processed in a timely manner.