You will turn 72 in October 2021, so you have to take your first required minimum distribution (RMD) from your IRA this year, 2021. The percentage that you have to take changes every year. It's not actually calculated as a percentage. You look up a divisor in a table and divide your IRA balance by the divisor.
The easiest way to determine how much you have to withdraw is to ask the IRA custodian to calculate your RMD for you. Many custodians will do it automatically, even if you don't ask. If you want to do it by hand, the rules for calculating your RMD are in IRS Publication 590-B, starting on page 8. If you search online I'm sure you can find an RMD calculator.
Since this year is your first RMD, you do not actually have to withdraw it until April 1, 2022. But if you wait until 2022 you will have to pay tax on two RMDs in one year, because you will also have to take your 2022 RMD in 2022. The deferral until April 1 of the following year is only for the first year. Also note that the deadline is April 1, not April 15.