If I have a child in 2021 how will I get the child tax credit?

You don't.  You get the 2021 tax credit when you file your 2021 tax return in 2022.

A child born in 2021 does not qualify you, retroactively for the 2020 Child Tax credit (CTC).   The child does qualify you for the expanded  ($3600)  2021 CTC.

There was some talk of the IRS making the advance monthly payments available for parents of 2021 babies, but so far that hasn't happened. See this IRS portal for updates https://www.irs.gov/credits-deductions/child-tax-credit-update-portal

You'll have to wait until you file your 2021 tax return, in 2022. 

There are two tax provisions you should look for as a parent of a newborn.

1. Recovery rebate. This is $1400 per dependent, and is the same as the stimulus payment that was made in March and April.  If your tax situation when you file your return is different then your tax situation last year and qualifies you for an additional stimulus rebate, it will be added to your refund.

2. Child tax credit. For 2021 only, the child tax credit was expanded to $3600 for child dependents under the age of five.  This credit is paid in advance for some taxpayers, and the credit is reconciled on the taxpayers tax return for 2021, which is filed between January and April, 2022. The IRS said they would create a website where parents of newborns could enroll their children for the advance payment, but this has not happened yet. You will claim the child tax credit on your tax return instead. If you did not get any advance payment or only a partial advance payment, the full amount will be credited against your tax or added to your refund.

For both the rebate and credit, you must claim the child as a dependent on your tax return and the child must have a Social Security number before you file an on-time tax return.  On time means that you file before April 15, or that you get an extension before April 15 and then file before October 15. If you don’t have the Social Security number when you file your tax return, you will not get the payments, and you will not get them even if you file an amended return later. So make sure you have the child’s Social Security number before you file your tax return and be sure to file the tax return on time.