This question already has an answer here:
I accidentally got rid of the menu/window at the right side of the screen which contains the modifiers button and the textures button (and a bunch of other stuff). Now it's just the Outliner window over there. How do I get the default one back? I tried clicking on the little button that contains a bunch of different window types (Properties, 3D view, etc.) but none of them were the right one.