Your environment is not setup correctly. Is this Windows, Linux, Unix, MS DOS 5.0? It would help to know what you are dealing with. You should probably look up how to create a listener for your OS, learn how to setup the environment variables for you OS. You will need environment variables for ORACLE_HOME, LD_LIBRARY_PATH, add $ORACLE_HOME/bin or %ORACLE_HOME%\bin to your PATH statement and finally ORACLE_SID. If you are using Linux/Unix there should be an oraenv script in /usr/local/bin, hence:

. oraenv
db name

Keep looking you will get the answer.