As @ik_zelf mentioned, your PATH variable will need to include the "bin" directory of your Oracle software install. This will allow you to execute commonly-needed commands/utilities like "sqlplus" and "lsnrctl" without changing directories. This is typically $ORACLE_HOME/bin
Your TNS_ADMIN variable needs to be set in a similar matter, pointing to the location of your tnsnames.ora, listener.ora, etc. By default, SQL Plus checks this directory (defined by TNS_ADMIN) for tnsnames.ora. This is typically $ORACLE_HOME/network/admin
If you want to connect to your database via TNS, your tnsnames.ora file (assuming a ORACLE_SID of "myDatabase", a host of "myHostname" and the default port) would look something like this:
(ADDRESS = (PROTOCOL = TCP)(HOST = myHostname)(PORT = 1521))
(CONNECT_DATA = (SERVICE_NAME = myDatabase)))
This would allow you to connect locally simply by doing the following from a command line:
Of course, as SQL Plus looks for the tnsnames.ora file in the location defined by $TNS_ADMIN, this will only function locally. To access this database via TNS (Transparent Network Substrate) from a remote machine, you would have to install the Oracle client and ensure that the TNS entry for myDatabase (as described above) is included in the remote machine's tnsnames.ora file as well.