The IRS has published the 2013 Circular E, Publication 15.  It can be found on the IRS website or accessed via the link below.

Make sure to start using the new tax tables as soon as possible but no later than February 15th.  Most software packages have already included updates to the tax tables so if you are running the most current payroll version you should be all set.

Remember that the employee portion of Social Security already to 6.2% for 2013.

http://www.irs.gov/pub/irs-pdf/p15.pdf