Welcome to the community!
You might want to read through the grapheneos.org website to familiarize yourself with the ideas and features. As for your user experience, I highly suggest you just keep it simple. Don't use user profiles, private space and many app stores at first. Install the sandboxed play store for getting your apps following the official guide. You can create a throwaway account following this guide from @fid02 in order to stay more private.
Over time, you will get more and more familiar with the extra control and detailed settings you have. But in the default settings, GrapheneOS will be much much more private and secure compared to any other popular mobile OS. Have fun and take it slowly. And don't hesitate to ask questions here in case you run into a roadblock.