In this Zip file you will find sql and plsql example by each topic.
 Download  example:
-->Click to download SQL
-->Click to download PLSQL
-->Click to download PLSQL ..MORE EXAMPLES

This will help you to understand the basic concept with small programs.
Note: Use scott user for the example to run. Generally the password for scott will be tiger.
If you don't know the password or unable to unlock scott or any other user, login as sysdba.
Following are the command to unlock and to create user in oracle database as sysdba.

Unlock and changing the password for database  user.
sqlplus>conn sys as sysdba
enter password. or you can just press enter where you have to give password.
sqlplus>alter user scott identified by tiger account unlock;

To create new user 
sqlplus>connect sys as sydba
sqlplus>create user demo identified by 123;
sqlplus>grant connect,resource to demo;
Now you will be able to connect as demo user;
sqlplus>conn demo/123