The IRS issued a statement on December 26, 2017 about how the tax changes will affect workers
The IRS is working to put together withholding guidance due to the tax reform bill signed into law on December 22. They hope to start putting out the initial withholding guidance in January, and employers and payroll service providers will be encouraged to make the changes in February. The IRS will be using the information from the W-4 forms you already filled out for your employer, so you don’t need to do anything yet. Use of the new 2018 withholding guidelines may mean that you begin see changes in your paychecks as early as February.