> How I can create new user for accessing data base. I commonly use
> SYSDBA for accessing my database.
>

Users are users of the server, not a given database. You can create a
new user using the Services API through whatever language you normally
use, or you can use the command line tool gsec that ships with
Firebird (a good place to start).

http://www.firebirdsql.org/manual/fbutils-gsec.html

You user will need to be granted access to roles, and those roles will
need to be granted specific rights to the tables, views, procedures,
etc within your database.

Adam