Set PATH variable in UNIX

The PATH environment variable is a colon-delimited list of directories that your shell searches through when you enter a command.

Program files (executables) are kept in many different places on the Unix system. Your path tells the Unix shell where to look on the system when you request a particular program. Having more directories in your path will reduce the number of times you get "Command not found" errors, but might put you at greater risk of activating a Trojan horse.
To find out what your path is, at the Unix shell prompt, enter:
echo $PATH
Your path will look something like the following.
/usr2/username/bin:/usr/local/bin:/usr/bin:.

You will see your username in place of username. Using the above example path, if you enter the ls command, your shell will look for the appropriate executable file in the following order: first, it would look through the directory /usr2/username/bin, then /usr/local/bin, then /usr/bin, and finally the local directory, indicated by the . (a period).

To modify your path
If you are using csh or tcsh, at the shell prompt, enter:
setenv PATH $PATH\:/dir/path

If you are using sh, ksh, or bash, at the shell prompt, enter:
PATH=$PATH\:/dir/path ; export PATH

In all cases, replace /dir/path with the directory you want the shell to search.

Note: The earlier entries in the path take precedence over the later ones. If you want the directories you add to your path to take precedence, in the examples above, replace $PATH\:/dir/path with /dir/path:$PATH .

To make these changes permanent, add the commands described above to the end of your .cshrc file (for csh and tcsh), .profile file (for sh and ksh), or .bash_profile file (for bash).

Source: http://kb.iu.edu/data/acar.html
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