Before you install Shopware 6, you should check that your server meets the system requirements.
The complete technical system requirements are described in the developer documentation: System requirements
If you are migrating from Shopware 5, the Shopware Migration Wizard will help you verify that the server of your current Shopware 5 installation and installed plugins meet the requirements to be migrated to Shopware 6. You can find the Shopware Migration Wizard for your Shopware 5 installation in the Shopware Plugin Store.
After you have installed and activated the plugin, reload the backend once. Click on the question mark symbol in the menu bar to find the Shopware 6 Update Check menu item.

The Requirements tab shows you which requirements for Shopware 6 your server currently already meets, and where you should configure your server before migration.

The tab Plugins shows you for your installed plugins whether they are also available for Shopware 6 and the configurations can be adopted.