A quick overview:
The region means nothing in terms of functionality, price performance etc. – they just show that the servers are physically located in different parts of the earth. Until you get going, and you want to be absolutely sure the same tornado won’t wipe out all your instances, you can ignore it.
The Flavor is also not important to start: The smallest one is the cheapest, and you won’t need any more till you have some traffic, so pick that.
The Security Group is (again) not important for now. It’s the ports you allow your server to connect to the outside world. Default is 22, 80, 443 and not much else worth worrying about yet. You’ll be able to secure shell in to your instance (port 22) and get a web server working right away (ports 80 and 443).
Install Image is so much fun I’ll save it for another post. Any of the linuxes will work fine.
Key Pair is also so much fun I’ll make it the topic of the next post…