cowboy's documentation is pretty poor and there's no good 'getting started with cowboy' thing i can point you at, but this is pretty good for erlang in general:
It walks through a simple "Hello world" application starting with creation from template (erlang.mk), and discusses static files, SSL, websocket.. A very good first guide in my opinion. Though it doesn't really cover other techniques like how to architect your application or interact with a database.