Ok ... on the purchase then ... it will not make much of a difference at all.
If there is a tenant already in place then you will report the income/expenses for the period you owed it on the 2021 return if you buy in 2021.
If there is no tenant in place when you buy it then the property is reported on the Sch E when you first make it available for rent what ever tax year that happens to be.
Read up on rental property rules in the IRS Pub 527 : https://www.irs.gov/forms-pubs/about-publication-527
