raincoat

So I've been looking at my missionary stuff, and things I need to bring, and one thing it mentions is a rancoat. I guess it rains a lot in Tennessee, I wouldn't know. Now, I have one slight problem. I've lived in a desert all my life, and it hardly rains in the spring or the fall. I've never used, let alone gone out and bought, a raincoat. I don't know what to look for, I don't know what might be a piece of junk, what might be good. Totally oblivious to the fact there were such things as raincoats, until I was told I would need to have one. All that I've ever used is a winter coat to keep out the cold Utah and Idaho air, and a fleece jacket, used for when it gets chilly in the spring and fall. Is there anyone out there who is more knowledgeable as to what should be in a raincoat who can clue me in? I asked a friend and they said, "just use a garbage bag with holes poked in it." As far as I know, that's basically all a raincoat is, just in coat format. :)