Knife throwing is fun, and even useful in some cases, but it's also dangerous -- and it's not a skill where you just pick up a knife and "know". Luckily, Instructables user bergerab is a master, and has a great guide to the art.
H starts the guide with making sure you choose the right type of throwing knife. If you're a beginner, he suggests rounded practice knives that have no real sharp edges, and a point that will stick into your target. He also notes that you should find a knife with as few perforations as possible (which can lead to shattering), and that fancy grips don't make a better throwing knife either -- just one that's harder to clean and take care of later.
From there, he walks you through some basic safety -- including finding a solid target like a large rotting tree trunk or other wooden target, and wearing hard-toed shoes (sadly too often overlooked). After the preliminaries are finished, you'll pick up a proper throwing stance and grip, and your first throw technique. The half-spin is usually the easiest for people to learn, and it doesn't require a lot of distance from the target, so you can practise easily.
Hit the link below for the full guide.
How to Throw Throwing Knives [Instructables]