The IRS website became available for filing 2021 returns as of Monday, January 24, 2022.

The IRS advises taxpayers watch for the following two letters:
Letter 6419 outlines the total amount of advance Child Tax Credit payments you received in 2021 and the number of qualifying children used to calculate the payments.
Schedule 8812 should be filed to claim any remaining credit or determine if you must pay back any of the payments you received in 2021.
Letter 6475 relates to the third stimulus payment that was available and how much you received. I am assuming that, again this year, if you did not receive this stimulus payment and were entitled to it; an adjustment will be made to increase your refund or reduce any taxes owed
Please watch for these two letters as they are important parts of your 2021 filing