How to switch between Oracle 11g and 12c on same linux
Check the /etc/oratab file. In there the ORACLE_HOME is defined in the second field. Make sure that both instances point to the right ORACLE_HOME. If you do an  . oraenv to change from one instance to another this scripts sets the correct ORACLE_HOME so you connect with the right binaries to start, stop, and access the instance. The file is normally also used to stat-up the instances at boot time.

Example of a line in the /etc/oratab file:

DEVA:/u01/app/oracle/product/11.2.0/db_2:Y


DEVA is the name (SID) of the instance, /u01/app/oracle/product/11.2.0/db_2 is the ORACLE_HOME, and Y indicates that the instance must be started at boot time.

You say that you have no DB background so be aware that you cannot run the same instance in both versions of Oracle.