How to use AS400?

AS400 IS A MANAGEMENT SYSTEM SOFTWARE USED FOR VARIOUS ADMINISTRATIVE JOB FUNCTIONS WITH A TEXT BASED USER INTERFACE.

you should have a session to connect to os400

Hello, at first please note that AS/400 is a "Computer" built by IBM, the proper updated family for the product is "IBM Series i" & "IBM Power i".
In addition to the "machine" itself, your question is very board, but anyway, if you are asking where to start you will need to have good information about the following topics: "OS/400: The Operating system you will be using", "CL commands: The command line commands you can use to access the OS functionalities", "RPGLE: The main programming language you can use to build good programs", "DB2: The main database structure of the files inside the AS400", "PFs, LFs and SFs: Physical Files, Logical Files & Spool files".
As you can see, you will need to learn lots of topics to know about AS/400, so if you specify what exactly are you up to, I can help you with where to look.
In general, you should know that what is known as "AS/400" is a very powerful and stable machine, using the OS/400 & RPGLE programming language, you can build amazing programs that can process large amount of data with spectacular performance.
And of course; working with the DB2 you will find the advantages of DB2 over other DBMS like SQL or Oracle.
Good luck.

OS400 is the control language to manange basic computer functions like DOS does with Microsoft PC's. The programming languages are COBOL, RPG, C, and Control Language. The AS400 uses relational databases. One of the tools to quickly get information from those datbases is Query400. DFU400 helps you quickly make changes to a file. I hopee this helps.

AS400 is an internal company software or known as a control language/script language used in companies to perform system level tasks like compiling programs, backing up data, changing system configurations, displaying system object details, or deleting them.
Commands are not limited to systems level concerns and can be drafted for user applications as well.