First of all don't panic, the IRS sends millions of notices and letters to taxpayers every year. An IRS notice will typically be about your federal tax return or tax account. Most of the letters will ask you to address a specific issue, however they may also send a letter requesting additional information.
If an IRS letter informs you that they have made a change or correction on your tax return, it is important to review the change carefully and it may even be necessary to discuss the matter with your tax advisor. Typically, if you agree with the change, then you should follow the instructions in the letter if they guide you to take action. If you do not agree with the change, then you or your tax advisor should write a letter to explain why you disagree. You should also include any documents that support your conclusion. There should be an address on a tear off portion of the IRS letter that will help you to send your response to the right location.
Most notices will not require a trip to the IRS or even a phone call. If you have a question and wish to call them there should be a phone number in the upper right hand corner of the notice with the phone number of the correct department.
Please note that it is unlikely that the IRS will call you without sending a letter first. They will not contact you by email or social media to ask for personal or financial information. If you are contacted and you are unsure if the request is legitimate you should contact your tax advisor before giving out any personal or financial information. If you and your advisor deem that the contact is suspicious you should report the suspicious activity based on the IRS guidance here.
Remember that receiving an IRS letter does not automatically mean you did something wrong. Take the time to carefully read the notice and most importantly follow up, as requested, in a timely matter. Don't be afraid to utilize your tax advisor and make sure to send them a copy of any letters when you receive them.