Usually the rule is to put one halide bulb every two feet apart of your tank length, but in your case I believe one would work as I don't believe an Oceanic 38 gal tank is over 3 ft long. I may be wrong. The bulb should be at least 8" above the water line. A tank height of 24" or more should consider 250W or 400W. Also you need to watch the heat build up in your tank. There are fans available to mount in your canopy. Some systems have built in fans to help with heat build up.
Depending on what you are keeping or wanting to keep, depends on the watts used. You should be able to get by with 10,000 K (kelvin) 175 watt bulb with this set-up. The higher the k # the bluer the bulb.
As for mounting the unit it will depend on what system you plan on buying or have purchased. Look at some of the members pics, canopies and lighting systems and see which ones best suit your system or design plans. Good luck with your project and keep us all posted on this project.