This is one option - starting from absolutely nothing! Just click "Create a New Game" in the left-side menu. Give it a title, and start writing rules! Please select a "type" from the menu provided as well - it'll make your game easier for other people to find.
Copy an existing game
Snag someone else's, copy it, and start editing! Find a game you'd like to build on, and click on the "Clone" tab. Give it a new title, and make some changes to the text if you like. Be sure to click "Save" at the bottom of the page. A new copy of the game will be created, owned by you - so it's all yours to edit from here on.
Your past revisions are stored by default, so you can always get them back, if need be.
There are also some templates and simple starter boards on the Common Resources page, if you want to start (almost) from scratch.
Comment on an existing game
All the games posted here are shared for public viewing, so take a look around! Find a game that looks interesting, and give it a spin. You can leave feedback on the game's page itself - so be sure to bookmark it to make it easier to find later. You can get to your bookmarks by clicking on the "My Account" link in the menu. Please keep your comments friendly and constructive, though.
Just bat ideas around
Check out the Discussions. Start a new one if something's on your mind. This is a great way to throw out starter ideas for other people to weigh in on, or to ask for feedback from others.
When a user is marked as being the author of a game or comment, you can click on their name to get to their user profile. From there, you can send them a message directly (don't worry, they don't see your email address), or send them a friend request. Friends will show up in the "Community" tab on your profile page (accessible through "My Account" in the menu).