Every person who appears on a tax return must have an SSN or ITIN.
You use form W-7 to apply for an ITIN. This is normally filed with your 1040; however, please read the two links below for the process.
In order to file your 2020 return, you will need to get ITINs for your spouse and kids, if they are to appear on your return. You will file form W-7 to apply for the ITINs. The usual way to file your W-7 is with your 1040 return.
Please see this TurboTax page on the W-7.
Also, please see How do I file with a W-7?