I hope in the future you don't want 8 hours to ask for help.
Before we can help you, we need to know more about what is happening.
oracle 12c database with sqlplus
- is this a database you have installed, or a database already running in your environment?
- this isn't the SQL*Plus forum, but connectivity issues would be similar if you were using SQL Developer
- what is your connection string, e.g. how are you attempting to connect?
for example, on my local system, I connect like so:
hr - username
oracle - password
localhost - the server/network address where the database is running
1521 - the port where the listener is hadnling connection requests for the database
pdb1 - the service name for the database i want to connect to