sometime we need to play in role of superuser like su or sudo, what is the Oracle Cloud Shell password for default account?
You don't have that. Use SSH with the private key and just sudo. Are you using OCI with Oracle Linux distribution?
EDIT: sorry, misread the issue. You can't 'su' in Oracle Cloud Shell. What do you need to 'su' for?
If you need more advanced features I would advise create a small (e.g. micro) instance, doing your stuff, and shutting down when not used anymore (you won't be charged when the instance is stopped).