if the database server and the sqlplus cleint are in the same machine,
a tnsconnectSTring is not  a must.
you can connect without using it like username/password.
if u intend to use it with a string then u have to create an entry in your $ORACLE_HOME/network/admin/tnsnames.ora file.
[b]1. first start your listner.[/b]
-- by default listner might be configured.
-- else do that manually by editing $ORACLE_HOME/network/admin/listener.ora file.

C:>lsnrctl start

LSNRCTL for 32-bit Windows: Version 8.1.6.0.0 - Production on 06-MAR-2003 08:03:06

(c) Copyright 1998, 1999, Oracle Corporation.  All rights reserved.

Starting tnslsnr: please wait...

TNSLSNR for 32-bit Windows: Version 8.1.6.0.0 - Production
System parameter file is C:OracleOra81networkadminlistener.ora
Log messages written to C:OracleOra81networkloglistener.log
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=itloaner1)(PORT=1521)))
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(PIPENAME=\.pipeEXTPROC0ipc)))

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=itloaner1)(PORT=1521)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for 32-bit Windows: Version 8.1.6.0.0 - Production
Start Date                06-MAR-2003 08:03:09
Uptime                    0 days 0 hr. 0 min. 2 sec
Trace Level               off
Security                  OFF
SNMP                      OFF
Listener Parameter File   C:OracleOra81networkadminlistener.ora
Listener Log File         C:OracleOra81networkloglistener.log
Services Summary...
  PLSExtProc            has 1 service handler(s)
  clone         has 1 service handler(s)
  local         has 1 service handler(s)
The command completed successfully

[b]2. add entries [/b]in tnsnames.ora manually 
  or
   u can use net8 assitent to create a local netserivce name.
C:>tail -10  c:oracleora81networkadmintnsnames.ora
STAFFA_PMWR =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = staffa.xxxx)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = pmwr.xxxxxxxxx)
    )
  )

[b]3. now connect using the entries.[/b]
for the above entry i would connect as
C:>sqlplus dbadmin@staffa_pmwr

SQL*Plus: Release 8.1.6.0.0 - Production on Thu Mar 6 08:08:54 2003

(c) Copyright 1999 Oracle Corporation.  All rights reserved.

Enter password:

Connected to:
Oracle8i Enterprise Edition Release 8.1.7.0.0 - Production
With the Partitioning option
JServer Release 8.1.7.0.0 - Production

dbadmin@staffa_pmwr >