Second, before you can run SQL commands you must have a database, so you will either have to create one or connect to an existing database. If you want to learn to create a database instance, the easiest way is to use the Database Configuration Assistant (DBCA). This is installed by default when you install the Oracle software. The DBCA will ask you questions about the database you would like created, and then it will create the database for you. You can then use SQL*Plus or Oracle Enterprise Manager to connect to the database and run SQL commands against it.
A good place to start is to read the Oracle9i Release 2 (9.2) Database Concepts guide. Note that you will need to create an account on Oracle's Technology Web site before you can review the documentation (registration on OTN is free). If you are installing Oracle 10g, the 10g documentation is located here
I would then suggest reviewing the "Creating an Oracle Database" section in the Oracle9i Release 2 (9.2) Database Administrator's Guide.
Once your database is created, you can then connect to the database and learn all about SQL commands.
This was first published in February 2007