What error messages you are getting ? What OS ? Whats your database name.

1)Check for ORACLE_PATH. Say if your Oracle Home is D:/Ora816. See your path should be having an entry D:/Ora816/bin. You can type in path at cmd prompt and see you have Oracle Bin set or not.
2) Try executing binaries from that location. Goto O_H/bin and try running SQL*Plus
3) See what you have in tnsnames.ora ? If your dbname is XYZ9. You should be having an entry in tnsnames.ora with the SID name and HOST name.
4)Try like this after making an entry in tnsnames.ora:
$ sqlplus system/manager@xyz1
$ sqlplus scott/tiger@xyz1

See the following example: My hostname is "nikhi" and dbname is "KRIS"
<font face=courier>
EXTPROC_CONNECTION_DATA =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1))
)
(CONNECT_DATA =
(SID = PLSExtProc)
(PRESENTATION = RO)
)
)

KRIS =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = nikhi)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = KRIS)
)
)
</font>