You have two ways:
First go to the particular path of Android SDK:
1) Open your command prompt and traverse to the platform-tools directory through it such as
$ cd Frameworks\Android-Sdk\platform-tools
2) Run your adb commands now such as to know that your adb is working properly :
$ adb devices OR adb logcat OR simply adb
Second way is :
1) Right click on your My Computer.
2) Open Environment variables.
3) Add new variable to your System PATH variable(Add if not exist otherwise no need to add new variable if already exist).
4) Add path of platform-tools directory to as value of this variable such as C:\Program Files\android-sdk\platform-tools.
5) Restart your computer once.
6) Now run the above adb commands such adb devices or other adb commands from anywhere in command prompt.
Also on you can fire a command on terminal setx PATH "%PATH%;C:\Program Files\android-sdk\platform-tools"