How to find host string?

I have just installed Oracle - DevSuitehome2, Oracle Database 10g Express Edition and Oracle Developer Suit - DevSuiteHome2. I am new to this Oracle forms, Oracle Reports, etc. During installation, it asked for the password. I entered it. It was written that use your password in SYS and SYSTEM Database accounts. After installation was completed I started SQL *Plus. I entered SYSTEM as username and my password.

                 ERROR:
                 ORA-12154 :TNS:could not resolve the connect identifier specified

I also tried SYS as my username but still same error. I opened cmd and typed:

                 C:\>sqlplus /nolog
                 SQL> conn / as sysdba
                 connected.
                 SQL> desc dba_users
                 NAME           NULL?           TYPE
                 ------         ------          --------
                 USERNAME                       VARCHAR2(30)
                 USERID                         NUMBER
                 PASSWORD                       VARCHAR2(30)
                 . 
                 .

                 SQL>select username,password from user_dba;
                 USERNAME                    PASSWORD
                 -----------                 -------------
                 SYS                         XXXXXXXXXX
                 SYSTEM                      XXXXXXXXXX
                 SCOTT                       XXXXXXXXXX
                 HR                          XXXXXXXXXX
                 . 
                 .

All passwords were written in complex form so to simplify it I thought to change any one of the password. So, I wrote :

                SQL> alter user HR identified by password;
                user altered.

Then I tried to use these while opening SQL *Plus. It did not work. I don't know what's the host string now. Can I get the complete information about users, passwords and host string ? I tried to search for tnsnames.ora file too but could not find out. All files, folders are all mess.

Usernames and passwords:

SELECT  USERNAME, PASSWORD
FROM    DBA_USERS;

Passwords are always encrypted so you will not get much out of it

host name:

SELECT sys_context('USERENV', 'HOST') FROM DUAL;

sys_context works great for many, many things:

http://psoug.org/reference/sys_context.html

If you have access to the command line you can use listener status to get some more information.

LSNRCTL> STATUS
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=net)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 11.2.0.0.0 -
Beta
Start Date                15-JAN-2008 12:02:00
Uptime                    0 days 0 hr. 5 min. 29 sec
Trace Level               support
Security                  OFF
SNMP                      OFF
Listener Parameter File   /oracle/network/admin/listener.ora
Listener Log File         /oracle/network/log/listener.log
Listener Trace File       /oracle/network/trace/listener.trc

Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=net)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=sales-server)(PORT=1521)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcps)(HOST=sales-server)(PORT=2484)))

Services Summary...
Service "sales1.us.example.com" has 1 instance(s).
  Instance "sales", status READY, has 1 handler(s) for this service...
Service "sales2.us.example.com" has 1 instance(s).
  Instance "sales", status READY, has 2 handler(s) for this service...
The command completed successfully

Source: http://docs.oracle.com/cd/E11882_01/network.112/e10835/lsnrctl.htm#CIHGHCGJ (Oracle Database Net Services Reference)