This is the very traditional recipe, but I've tweaked the quantities to make it much easier to shop for and work with. From start to finish you can have this done in half an hour or so, and the technique really is beginner-level easy.
Unfortunately, if you don't live in an area with a Chinese grocery store, ingredient availability will be a problem. The hardest items to find are usually the kha (also called galangal) and the kaffir lime leaves, but if you can't find them PLEASE don't let that stop you; it's still excellent even without them. Everything else listed here is very easy to find, between the Chinese grocery and regular grocery.
This is one of the national dishes of Thailand, and is usually found at Thai restaurants, but this recipe is better than ANY I have ever had in a restaurant. I can't claim any credit though; I'm just passing along a very good recipe. WARNING: this soup is totally addictive!