the best way to map a cube is not to. When a shape is generated each vertices is assigned a uv coordinate - the reason for UV mapping by projections is to put order into complex model. - However a cube has perfectly fine UV coords already. So, generate your cube and assign it a material then select it and look in uv editor, you should have large UV's. Select UV picker overall the uv's and scale down so they all fit in the top right hand square, snapshot your uvs and paint away.

Nice and easy. I recommend buying Learning Maya 6 Foundation - you can get it from amazon quite cheaply, it is exellent for starters then move onto maybe doing some of the simplymaya stuff. Dont try to run before you can walk.