As a root user, where do I get all IAM user credentials?

The root user isn't an IAM user.

If you want to create an IAM user with full privileges then login to AWS Console as the root user, select the IAM service and add a new user (called it something like admin or whatever you prefer) and then attach the AdministratorAccess policy.

This new IAM user will have full administrator privileges. Be aware that this is not a recommended way of working. Instead the concept of least-privilege permissions is to grant users the permissions required to perform just the task(s) they need to, and no additional permissions.