Oracle 11g r2 standard edition installation - set environment variable PATH and create tnsnames.ora
Normally you use the oraenv script to set you environment correctly for a given ORACLE_SID. The ORACLE_SID and ORACLE_HOME combinations are maintained in /etc/oratab or /var/opt/oracle/oratab which is read by the oraenv script that should be sourced (called with a .)

What oraenv by default sets does is make sure that PATH contains at least $ORACLE_HOME/bin

If you want to have some control over where your SQL*net files are located it is smart to create a TNS_ADMIN environment variable that points to the directory where amongst others tnsnames.ora is located. The tnsnames.ora file is most important to be able to point to a - local or remote - database using SQL*net.

If you only access your database from the same machine as where the database is running and you do not access an other database, a tnsnames.ora is not needed, just convenient. Normally we have a tnsnames.ora that contains lots of database on the site.