Error ORA-12505 when trying to access a newly installed instance of oracle-11g express
you should create at least one database user that you will use to create database objects. A database user is a type of database object: a user is associated with a database schema, you connect to the database as a database user, and the database user is the owner of any database objects (tables and so on) that you create in the schema associated with the user.

For example, to create a database user named chris(For Example). Follow these steps, using the command line:

Display the SQL command prompt window. For example, on Windows, click Start, then Programs (or All Programs), then Oracle Database 11g Express Edition, and then Run SQL Command Line.

Connect as the SYSTEM user:

Type: connect
Enter user-name: system
Enter password: 
Create the user. For example, enter a statement in the following form:

SQL&gt; create user chris identified by &lt;password-for-chris&gt;;


Grant the user the necessary privileges. For example:

SQL&gt; grant CREATE SESSION, ALTER SESSION, CREATE DATABASE LINK, -
  CREATE MATERIALIZED VIEW, CREATE PROCEDURE, CREATE PUBLIC SYNONYM, -
  CREATE ROLE, CREATE SEQUENCE, CREATE SYNONYM, CREATE TABLE, - 
  CREATE TRIGGER, CREATE TYPE, CREATE VIEW, UNLIMITED TABLESPACE -
  to chris;


Optionally, exit SQL*Plus (which also closes the command window):

SQL&gt; exit


SQL Developer as an Alternative for Creating Database Users:

If you have experience with SQL Developer, you can use it instead of the command line to create a database user, as follows:

Create a database connection for the SYSTEM user.

Open that database connection for the SYSTEM user.


Right-click the Other Users node in the Connections navigator under that connection.

Select Create User, and specify the necessary information. (Under System Privileges, grant ALTER SESSION, CREATE SESSION, CREATE DATABASE LINK, CREATE MATERIALIZED VIEW, CREATE PROCEDURE, CREATE PUBLIC SYNONYM, CREATE ROLE, CREATE SEQUENCE, CREATE SYNONYM, CREATE TABLE, CREATE TRIGGER, CREATE TYPE, CREATE VIEW, and UNLIMITED TABLESPACE.)


I am also attaching the screen shot with example , which is given by Oracle BOL for SQL Developer connection.



For your ref Here And Here.