In the letter you received from the IRS there should instructions on what you need to do if you disagree with their assessment of your taxes owed.
You can also set up an appointment at a local IRS office.
See this IRS website for local IRS offices - http://www.irs.gov/uac/Contact-Your-Local-IRS-Office-1 or call 1-844-545-5640 to set up an appointment