DJ who? Aside from the text, I think it's of utmost importance to recognize unwanted grainy artifacts created from stretching/distorting. Ask yourself, "Would I see something like this extremely out of place on an Apple device?" Unless you're sweet bro or hella jeff, never expand, only shrink.
Learn in-depth and know how to function: Gradients, brushes, filters, layer blending options, shapes, masks, colors, complementary colors, hues, saturation, brightness... the list goes on
Not necessary, but aesthetically moral:
http://en.wikipedia.org/wiki/Rule_of_thirds
To those who complain about how tutorials suck--why would you put down something that has direct invaluable information in the style of a learning environment? Either you have ADD or believe that there's some magic Photoshop button going to do it for you.