TEXTURING.
To texture, go to the grid above, and right mouse click on it. Then pick 'Split Area'.
You should see a new sortoff window. Click on the this thingy and go to UV/Image editor.
You should now see a blank/gray screen with lots of cubes in it.
At the toolbar, click on the arrows and a window will popup with the background image you've used. Click on the image.
You should now see your weapon on both screens.
Go to the first one, select all vertices in Edit Mode, click U, then click on 'Project from View'.
You should now see your selection on the UV/Image editor. Scale ( S ) and Grab ( G ) and Rotate ( R ) till the selection fits the image.
Go back to your first screen, click on the button RIGHT next to the modes button ( edit, object etc. ) and click on 'Textured'.
Your weapon is now textured! Have fun with it.
IF YOU DIDN'T UNDERSTAND THIS, SEARCH 'STYX's BLENDER TUTORIAL. IT'S EXPLAINED THERE TOO.