To log in to your AWS RDS database (Oracle in my case) you need login credentials, but what are these for a newly created DB? The password is the master user password you entered during DB creation and which you can change via the Console.
To find out the master user name:
aws rds describe-db-instances --output table --query 'DBInstances[*].DBInstanceIdentifier'
aws rds describe-db-instances --db-instance-identifier <an identifier from the previous table> \ --query 'DBInstances[*].MasterUsername'