What do I do if the IRS notice asks for certain information from a return I filed?
You are being audited. The IRS can perform a "mini" audit where they ask for a small number of items on your return, or they can ask for an explanation and substantiation of major deductions. IRS agents have a backlog of cases, so by providing the IRS agent with the requested information and an explanatory letter, your case is more likely to close quickly and without change.
Nonetheless, it is important that you allow yourself enough time to adequately prepare for the meeting. Do not be intimidated into scheduling the meeting before you are ready. Request more time if needed to document what you have done and gather receipts. While you must keep the Agent informed as to when the information will be available, the audit cannot be postponed indefinitely.
If you are uncomfortable about going solo, or at all, you'll want the help of a qualified tax advisor, possibly even to represent you before the IRS.