The upgrade rollout plan is still being finalized and is scheduled to begin in late 2021 and continue into 2022. Specific timing will vary by device. And certain features will require specific hardware—see https://www.microsoft.com/windows/windows-11-specifications to learn more.
To see if your PC is eligible to upgrade, you'll soon be able to download and run the PC Health Check app. Or, after the upgrade rollout begins, you can check if it's available for your device by going to Settings > Update & Security > Check for updates.