What is the main schema that can be used to develop oracle database for an application. actually, i don't understand at which place i can build my oracle database is there is public schema that can be used by multip user to build the database ?
DBA should create a new user in the database; user is identified by its username and password.
DBA will also have to grant certain privileges to you, such as
without privileges, you can't do anything.
Once DBA provides login credentials to you, you'll be able to establish connection to the database and ... well, do something - create tables, insert data into them, select data etc.
User + all its objects make a schema. That should be your playground to develop an application.