For problem 1, try entering ‘sqlplus internal’. You need to be logged on as the oracle account do do this, if you’re on UNIX. If you’re on Windows, you’ll need to do it from a DOS prompt. Make sure you set your ORACLE_HOME and ORACLE_SID environment variables.
For problem 2, the Oracle data dictionary views are documented in the oracle database documentation. There’s too many of them to list here. You can download this doco from technet.oracle.com.