On January 11th, the Internal Revenue Service released Notice 1036, which includes the updated 2018 tax withholding tables. You can see the full notice here.
The IRS recommends that employers and payroll service providers begin implementing the tables as soon as possible. However, the official deadline for the implementation is February 15th- one month from today.
At this time an updated form W-4 has not been provided. The 2017 W-4 should be used until a new one is issued, which is expected shortly.
The following are the updated tax tables which were released in December.