how to make a health bar above the enemy using plane object in c #, please help me
If you want to use a gameobject, you could make it a child of the enemy object and script it to always face the camera.
You could also use the GUI-system. There is a function to get screen coordinates from a 3d-position.
http://docs.unity3d.com/Documentation/ScriptReference/Camera.WorldToScreenPoint.html