I need to download the video tutorial of pl/sql, please someone should tell me the site.

In fact, i could always log in with this password (hr/hr) from sqlplus and even from command prompt (sqlplus and then the password hr/hr).
But it could not give me the room to create or alter the user. The result has always being "insufficient privileges". I do not know where the problem is from, although, am using Windows 7 OS with Oracle 11g release 2

It returns this result even when i connect as system or sys

SQL> connect /as system
SP2-0306: Invalid option.
Usage: CONN[ECT] [{logon|/|proxy} [AS {SYSDBA|SYSOPER|SYSASM}] [edition=value]]
where <logon> ::= <username>[/<password>][@<connect_identifier>]
<proxy> ::= <proxyuser>[<username>][/<password>][@<connect_identifier>]

SQL> conn /as sysdba
ERROR:
ORA-01031: insufficient privileges

Warning: You are no longer connected to ORACLE.
SQL> connect /as system
SP2-0306: Invalid option.
Usage: CONN[ECT] [{logon|/|proxy} [AS {SYSDBA|SYSOPER|SYSASM}] [edition=value]]
where <logon> ::= <username>[/<password>][@<connect_identifier>]
<proxy> ::= <proxyuser>[<username>][/<password>][@<connect_identifier>]

I DID SOMETHING LIKE THIS
ALTER TABLE customers
ADD CONSTRAINT customers_customer#_pk PRIMARY KEY

I JUST CREATED A CONSTRAINT TO THE CUSTOMERS TABLE I DID NOT REFERENCED IT TO ANY TABLE AT ALL.
ACTUALLY, I WANT TO DELETE IT SO THAT I CAN RECREATE A NEW CONSTRAINT TO THE TABLE....