You can start preparing your tax return any time. Some forms are not finalized so you may not be able to e-file for awhile yet. The IRS is not accepting returns until January 29.
Employers, banks, brokers, Social Security, etc. have until the end of January/early February to issue the documents you need, so wait until you have everything before you file.
https://www.irs.gov/newsroom/2018-tax-filing-season-begins-jan-29-tax-returns-due-april-17-help-avai...