cd /path/to_whatever (a lot like DOS)

cd $ORACLE_HOME/rdbms or whatever

What are you environment variables?
env|more

What from Oracle is running on the box?
ps -ef|grep ora

How to become the user 'oracle' (or whatever the name is)
su - oracle

Where is the home directory for 'oracle'
As oracle, just enter cd and hit return, then enter pwd

You can also look in /etc/passwd to see oracle's home directory

Where the files are:
same commands in SQL*Plus as you use on Windows