Android is based on Java so
First you need JDK(Java Development KIT) installed in your machine and it needs to be configured in the Environment Variables.
To check if JDK is already installed in your machine. Open the command prompt and type “javac”

Then press enter. If you see a screen like this

That means you have JDK installed and configured.
else go to
http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html

And download the JDK suitable for your pc version(The latest version changes with years this Screen shot is taken on 2014 September)

After you have downloaded it and installed it copy the path of the bin folder in side the installation folder. Installation folder path will be normally
“C:\Program Files\Java\jdk1.8.0_05\bin”
Depending on the version of Java u have installed the folder name and depending on the Windows version the program folder name also changes.

Now Go to Control Panel-> System and click on Advanced System Settings

Step 1
You will see a new Window called “System Properties” Pop up

Step 2
Then in that Windows you will see a Button Saying “Environment Variables”, click on it

Step 3
It opens a new window called “Environment Variables”

Note: Before Going to Step 4, check if the “User Variables for ‘Your PC user name’ “, already got “path” in the list, if it was not there follow Step 4 else skip to Step 5.2

Step 4
In that window under the “User Variables for ‘Your PC user name‘ “Click the New Button

Step 5
Now you will see a new window “New User Variable”, there on the ‘Variable name‘ section type “path”, and on the ‘Variable value‘ section paste the java sdk path you have copied earlier

Step 5.2
If there is already a path variable, select that section and click on the “Edit”, and without doing anything  in the ‘Variable name‘ section, go to the ‘Variable value‘ and add a “;” next to the existing value and paste the java sdk path you have copied earlier.

Step 6
After adding the value, click ‘OK’ , then again ‘OK’ on the “Environment Variables” window and finally click  on the ‘Apply” button on the “System Properties” window.

Now you will be able to get the result I have got earlier when typing “Javac” on the command prompt.