1. edit your tnsnames.ora file available at
   $ORACLE_HOME/network/admin
say for this entry,

STAFFA_CPUB =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = *******)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SID = cpub)
    )
  )

staffa_cpub = is the connec string
host        = name of your machine or IP
sid         = your datbase sid

for this tnsnames.ora config,
i would connect like
SQL*Plus: Release 8.1.6.0.0 - Production on Thu Oct 31 08:08:17 2002

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

Connected to:
Oracle8i Enterprise Edition Release 8.1.6.0.0 - Production
With the Partitioning option
JServer Release 8.1.6.0.0 - Production

SQL> connect dbadmin@staffa_cpub
Enter password: *********
Connected.
SQL>