If you are referring to the Child Tax Credit that will begin getting paid out in July 2020, Kiplinger's has an excellent calculator you can use.
In general, you may be eligible to receive up to 50% of the child tax credit paid out through the months of July - December based on your filing status, number and age of dependents claimed on your tax return and your income. The IRS will use information off your most recent filed tax return, presumably your 2020 return, to automatically calculate this. This is an ADVANCE of the credit you would normally receive on your 2021 tax return when you file next year.
The IRS is supposed to create a portal allowing you to update your information and/or opt out of receiving these payments.
