Rendering Error when I press F12 in Blender I'm a newbie in blender.. and everything was going fine but I don't know whether I clicked something or what, now when I render it gives the error: "No render output note in scene".

I don't even know what this means it must be something very easy that I overlooked.
In the compositor editor type (enabling "Use Node") you should see two nodes one attached to the other. A Render layer node attached to a composite node. The message you are talking about is shown when the composite node is missing. 

If it is missing press Shift + A -> output -> composite. Plug the render layer image output into the composite image input.