SQL*Net configuration on the PC using newer versions of the Oracle Client Software is very easy. The best (and easiest) way of achiving a fully working client installation is to use the SQL*Net Easy Configuration tool supplied by Oracle.
This toolhas a wizard type interface to take you through the installation of the tnsnames.ora and sqlnet.ora files.
Select "Add Database Alias" and enter a name for the alias when prompted. This alias is the name you will refer to the database instance as, and as such should be the same as the instance name (orcl in this case).
Select TCP/IP as the protocol, and when prompted the hostname of the machine hosting the database and the instance name of the database.
However, if you do not have the SQL*Net Easy Configuration Tool then don't worry. You can simply create the tnsnames.ora and the sqlnet.ora files in the $ORACLE_HOME/network/admin directory on the client exactly as they are on the server. This will provide an alias the same as on the server (always a good idea anyway).
UNIX clients are not very different that windows clients. If you have the Network Manager from Oracle then user that in the same way as above, if not then you can, again, just use the same configuration files as the server in the $ORACLE_HOME/network/admin directory.