How do I set or change the PATH system variable?

Windows 2000/XP users may set their path by right-clicking on ‘My Computer’ and selecting ‘Properties’. Under the ‘Advanced’ tab, there is a button that allows you to set the ‘Environment variables’. Click on this and alter the ‘Path’ variable so that it also contains the path to the Java executable. For example, if you have installed Java in c:\jdk and your path is currently set to C:\WINDOWS\SYSTEM32, then you would change your path to read C:\WINDOWS\SYSTEM32;c:\jdk\bin
When you open a new command prompt, it will reflect these changes and allow you to run java programs by typing “java”. If you have installed the SDK, then you will also be able to run “javac” to compile stuff.

For Windows Vista:

1. Right click “My Computer” icon
2. Choose “Properties” from context menu
3. Click “Advanced” tab (“Advanced system settings” link in Vista)
4. In the Edit windows, modify PATH by adding the location of the class to the value for PATH. If you do not have the item PATH, you may select to add a new variable and add PATH as the name and the location of the class as the value.
5. Reopen Command prompt window, and run your java code.

Setting Path on Solaris and Linux
To find out if the java executable is in your PATH, execute:
% java -version

This will print the version of the java executable, if it can find it. If you get error java: Command not found. Then path is not properly set.

To find out which java executable the first one found in your PATH, execute:
% which java

Below are the steps to set the PATH permanently,
Note: We are here giving instructions for two most popular Shells on Linux and Solaris.
Please visit link below if you are using any other shells.
Path Setting Tutorial

For bash Shell:

1. Edit the startup file (~/ .bashrc)
2. Modify PATH variable:
   PATH=/usr/local/jdk1.6.0/bin
3. export PATH
4. Save and close the file
5. Open new Terminal window
6. Verify the PATH is set properly
   % java -version

For C Shell (csh):

1. Edit startup file (~/ .cshrc)
2. Set Path
   set path=(/usr/local/jdk1.6.0/bin )
3. Save and Close the file
4. Open new Terminal window
5. Verify the PATH is set properly
   % java -version

http://www.jibble.org/settingupjava.php;  http://www.java.com/en/download/help/path.xml