Assuming that it has a drainage hole and you stand it on something to raise it a little from the ground it shouldn't really need anything else. Cover the drainage hole with something to prevent the soil washing out - a piece of broken pot, an old piece of J-cloth, a bit of broken slate etc. If you want something more try broken pieces of polystyrene packaging for example.

If it hasn't gt a drainage hole it would be best to make one. I'd be tempted to line the sides with polystyrene as well for heat insulation as metal can get pretty hot in the sun.

I would place nothing in the bottom of the pot. Soil washing out will be self limiting as a network of roots start running across the bottom. If you wish, you can set the pot on top of decorative river rock to keep it a bit above the surface that it is sitting on.

I will assume that the company that made the metal PLANTER ( the questioner stated it was a planter but I called it a pot in my answer) made it with drainage holes. Some will place something in the bottom of the planter, my preference is to place something outside the bottom of the planter. I am also assuming an outdoor setting since the questioner did not specify in or out. Now if it is a metal Pot without holes as you say, I have a recipe for a nice stew.....:). Well, in any case if the planter does not have holes in it, one will have to set the plant in a smaller planter that does and then set that planter into the metal planter which does not have holes which means stagnant smelly water will collect on the bottom no matter what the drainage material is (even charcoal will fail) and then the whole smelly mess will have to be dumped out every week, clean drainage material placed in and who wants to go through that:( What? It's 3:32am as I make this reply and my pomerainians are barking to go out for their compfort walk. Good thing I live in a low crime area. Have a great day Snoop!

You can't always assume things though and I was playing safe. It is possible to drill holes though, even in metal.Putting another pot inside would as you say be a recipe for disaster. Covering the hole as I suggested prevents soil washing out in the early stages before the network of roots takes over. I got the J cloth idea from Bamboo and find it very useful. Some pots have quite big holes - some of mine do anyway.