Okay, here's how we unwrap a cube so that we don't get any whonky image output when we view the image mapped onto it.
Go into edit mode, select the entire mesh, and then hit U > Cube projection. This is what to do if you want the same texture on every face, which is what I assume you are going for. However! if this is not wht you are going for, there is always ...
So who here has seen a baseball? Raises hand "me!..Me!"
You know how those two white panels are stitched together interlockish-ly? how if you were to cut the red seams they'd Pop out and you could lay them flat? That is what we do when we unwrap things, but we still need seams.
If your cube were made out of paper, and you folded it to get it into a cube shape, what did the paper look like before it was folded?
Like this maybe?
Well, what you need to do, is select all the edges that need to be seams and Ctrl + E > Mark Seam. this will make them BRIGHT RED!!!!
It also means that those edges are now like the seams on a baseball, and all you have to do is U > Unwrap.