As a new player, you would need to join a game, you can find games in the LFG Tool under the Games menu, Join Game. You'll want to look for games that are welcoming of new players. If you have friends playing already, they'll need to send you an invite to their game. While waiting to get in a game, you should create one on your account (Games menu, Create New Game) and start playing around. There's a Tutorial that will pop up the first time you enter a game, which you should go through. It MAY be broken part way through right now because of a recent update, but don't let that stop you from going through what you can of it. Beyond that, start poking about in the tools and creating Handouts, Characters, etc. Practice with rolling dice and get familiar with the drawing tools and general navigation.
