Here is a very simple, generic tutorial I wrote up for a dirty metal texture. I tried to keep it pretty easy. This is just a starting point... I'm building off this basic texture and adding things like scratches, errosion, paint and maybe some crate support bars. So if you follow along with this tutorial, be sure to keep it so you can pick up where you left off.
Step 1: Open a blank document. I made mine 512x512 but you can work at 256x256 if you like. Fill the background with a low-saturated color. I used a grey-green
Step 2: Open a new document with a freebie metal texture to use as your base. Do a select all -> edit -> define pattern. Click back to the original piece and put a new layer above the green. Do an edit -> fill -> pattern. This is probably the easiest, quickest way to start out. If you want, you can use the same texture I did. Here is the link
Step 3: The texture I chose repeats pretty badly, so I want to clean that up some. Select the rubber stamp tool. Hold down alt and click a section of your image to copy from. Lift off the alt-key, and with a soft-edged brush start painting in a new area. This picks up the image from the old place and "pastes" it over into the new one. You'll probably want to fiddle around with this to get it the way you like
Step 4: On the metal layer, decrease the textures saturation by about 60%. I also chose to change the hue a bit and make it a little greener. Set the blending mode to "overlay" and bump the opacity down the around 90%. Go to filter -> render -> lighting effects. Fiddle with the settings to get the effect you want, here are the settings I used:
Step 5: At this point I usually run a noise filter just to make it a little grungier. Be sure to select monochromatic. After doing this increase the contrast of the texture by about 40% to make it pop a little more (or you can just duplicate the layer to get a similar effect) Now you should have a pretty generic dirty metal texture to work with.