Excuse me, I had installed oracle in my PC for doing assignment, but I am a new user of oracle. I have some problems when trying oracle.

The first one is, how to create database in oracle? Actually I know the statement is "CREATE DATABASE xxx", but I don't know why I can't create it, the database said I don't have enough authority. When installing Oracle, two a/c passwords must be satisfied, sys and system. But when I want to use sys a/c to login, I don't know why the password is wrong, so I use system a/c to login, but it say not enough authority to create database, what can I do for this situation?

The second one is, how to use various databases using sql commands in oracle.., as we can do in mysql(use database <database_name>)

Thank you for those members.