The first step to unwrapping a box is to actually draw the box, after this you need to convert it to an editable poly.
After this you go to the modifier list and hit the drop down menu and go to Unwrap UVW.
Then you select edit and it brings up a new screen which is the edit UVW’s screen.
From this screen if you select the select faces button and then select the whole map.
Once you have selected the full map you go to mapping and then unfold mapping and it brings the box up like it would look if unfolded a box created from carboard in real life.
Then you go to tools and render UVW template and from here you can render and export your map as a JPEG to add textures to it.
When you’ve exported your map you can texture it in photoshop and then into CrazyBump to give it the little bumps and details to make the object feel and look real, rather then it just been completely flat.