Drilling through metal is a lot harder than drilling through wood, and you'll need a tougher drill bit. Take a look at the drill bit you've got now -- it probably tapers to a point, which is the sign that it’s not right for steel. Go to a hardware or home improvement store for a cobalt bit that is designed specifically for drilling through steel. You want a cobalt bit, as it's a type of high-speed steel (HSS) that has more cobalt in it and is strong enough to cut through hardened steel. This time, you'll notice that it has a blunt tip, not a pointed one.
Before you start drilling, put a few drops of oil on the metal to lubricate the drill bit and reduce the amount of heat produced during drilling, which lessens the chance of dulling the drill bit. For this reason, you'll also want to drill slowly to prevent overheating. Put a block of wood under the metal, so when the drill bit penetrates the steel, it won't get dulled by whatever other surface else you're working on.
If you've already drilled a hole and you want to make it larger, fold a small piece of emery cloth in half, with the abrasive side facing inward, and put it over the hole. The advantages to drilling this way is that you'll be cutting down on the noise level while you work; in addition, you won't get a jagged hole. Drill right through the emery cloth with the wider drill bit, and you'll see a nice, smooth hole.