If you agree with the IRS notice, just follow the instructions on the notice itself.
If you need to communicate with the IRS, you usually can call the IRS at the phone number listed on the IRS notice or the general IRS phone number 1-800-829-1040. However, keep in mind that the IRS employees answering the phone often have limited training and can give wrong or incomplete advice. If calling doesn't work to resolve your problem or if you have a more complex problem, then you should write a letter to the IRS.
If you owe money and can't pay the IRS the full amount, then you generally won't send a letter; rather, you would send Form 9465, Installment Agreement, or Form 656, Offer In Compromise.