Starting sqlplus

Hi,
I am a new DBA in training and I'm having trouble starting my sqlplus. I installed EM and I'm trying to find out my current ORACLE_SID and then change it. When I type in:
$ sqlplus

I get an error message of bash: sqlplus: command not found

Am I in the wrong bash? Or do I need to be in a certain directory? Please help, I hope this isn't too dumb of a question. Thanks in advance for your help.

sqlplus isnt in your path

type echo $PATH and you will see that $ORACLE_HOME/bin is not there, also best to have your ORACLE HOME set as well as the SID