1. Don't panic, you haven't even opened it yet and you have to do that first to see what it is about.
Open it and read it from start to finish so that you know what they want. Most likely it is about some detail concerning your tax return that you can solve easily.
2. The instructions should be clear so compare the notice from them with your tax return if it is about that and solve the situation
If it isn't clear enough contact the IRS and find out more.
3. If they have corrected your return and show that information and you agree with it most likely you don't have to reply unless you owe more money. Pay it.
4. If you disagree with the correction that the IRS made, you still have to respond.
Write to them and explain why you disagree.
Include the documents that you want the IRS to read and consider. Add the bottom tear off section from the notice they sent to you.
Mail the information to the IRS address shown in the lower left corner of the notice. It usually takes up to 30 days for the IRS to reply.
5. Most times people don't have to call or make a visit to the IRS office but when you call use the phone number in the upper right hand part of the notice that they sent to you.
Be sure to have your tax return and other correspondence in hand for the conversation.
6. Keep copies of any correspondence together with your tax records.
For information from the IRS concerning notices and bills, look at Publication 594
Roger Chartier - The Author