> Now I am looking for a step-by-step documentation, that will get me
> through the DB creation, creation of TABLES and manupilation of DATA.

There is not much that is Firebird specific here. Have a look for SQL
101 tutorials for the syntax of create table and other DDL (data
definition) as well as some basic DML (data manipulation).

>
> I am really a beginner, so maybe someone can help me to set up a
> simple Database using the SQLI or maybe some other ways to create
> databses in a quicker way.

There are many tools that can convert data models into .sql scripts,
one of those is probably a good place to start. There are also many
tools that allow you to browse and manipulate the structure of your
database (eg Flame Robin or Database Workbench)

'The Firebird Book' by Helen Borrie is worthwhile if you are working
with Firebird, but your question seems to be more looking for SQL help.

Adam