From the RSW home page, click the "Play" button at the top of the page. If there are any pending games available currently ready for signup, they will be listed, and you can pick any one of them to sign up for.
If there aren't any games listed, or the none of the listed games are satisfactory, feel free to suggest a new game to add to the list. When you suggest a new game, you get to specify the terms of the game; and when other players sign up for the game, it will begin.
Please don't sign up for a game without the intention of playing it all the way to completion. It spoils the game for other players when people sign up for a game and then drop out after a few turns. If you are not sure whether RSW is for you, then you may want to get together with a friend or two, and start a private game just between yourselves to try it out. You can even create a one-person game just for yourself, if you prefer, just to try out the mechanics of the game.