ERROR: ORA-12154: TNS:could not resolve service name
only appears when you don't have an instance of a database or you don't have the listener on.
Make sure you have the DB online and the listener.
You can view the listener status with: 'lsnrctl status' and start it with 'lsnrctl start'.
When the listener is running, you can enter as sysdba (defined when the DB is installed) and type an 'startup' command to start the database you have define. Use 'startup db_sid' if you have more than one.