A question on painting on metal surfaces reminded me of some procedures and products that I used to great success on outside porch lights and driveway pole lamps. Painting on metal is not difficult, but it is important that the surface be free of rust, flaking paint, and dirt.
Start out by inspecting the surface to be painted. Are there any rusted spots? If so, they need to be removed with a wire brush or if small enough, medium grit sandpaper might work. Use both gently though so as not to gouge the metal. Is the quality of the current paint decent, with no flaking or peeling? If so, great. If not, then the paint will need to be wire brushed/sanded as well. Be sure to do these two activities in a well-ventilated area and use a mask and safety goggles to prevent eye injuries as well as breathing in any of the powder that you remove from the metal. If the spots are large enough, you may have to put a rust prevention paint on them prior to cleaning and painting. Depending on what you are painting, you can find rust stopper at your local hardware and/or auto supply store. It is toxic however, please use caution and follow manufacturers warnings and directions.
Once your piece is smooth and flake/rust free and primed, you can move onto the next step: cleaning. I like trisodium phosphate (known as TSP in hardware stores) for a quick rubdown. Use gloves and the TSP sparingly as it can sometimes cause more damage than not. My experience has been that it does provide a nice matte surface after cleaning that allows paint to adhere well.
Visit your local hardware store and ask for metal paint. It comes in a variety of colors and sheens so be sure you know what you are after. A quart is usually more than enough for most things unless you are painting something super large. Also, it allows you to try out the paint first without spending a ton of money. When applying the paint, it is best to use disposable foam brushes. This is for two reasons, first the foam brushes can get into a variety of small detailed spaces that lamps and elaborate metal bedframes have in abundance; and secondly they don't leave brush marks.
Follow the instructions on the can for drying times, etc. Try to place the items somewhere where the wind and weather won't affect them before they're completely dry. If this is not possible, choose a day with moderate weather and no wind to complete your project. You don't want detritus of outside stuck on your bedframe now do you?