Select the cube (Left mouse button if you have changed the configuration, Right otherwise)
Name the selected object “Box” (press “n” for the “name” panel and type in “Box” under Item)
Now is a good time to save your Blender file, give it a name like “box.blend” and keep all your files for this model in the same folder
Switch to Edit Mode (The mode selector currently shows “Object Mode”)
Switch to Edge Select Mode (Three buttons on the bottom bar of the window: vertex, edges and polys)
Multi-select (hold SHIFT) the “seam” Edges of the Box (for a fold-out cube)
Mesh→Edges→Mark Seam
Split main window (Drag the upper right corner of the 3D window to the left, revealing a second copy of the 3D window)
Select the UV/Image Editor view for new window from the view selector in the lower left corner of window
Select your object again in the 3D view (press “a” twice to select the whole object)
3D Window: Mesh→UV Unwrap→Unwrap (you should now see the fold-out texture from the cube appear in the right UV/Image window)
UV Window: UVs→Export UV Layout..