If you want to interact with an object (sitting down, for example), single left click on the object. If you want to play an animation, single left click on your avatar, and select an item from the animations tab (different lists of animations are available depending on whether you are sitting or standing).
And ... that's actually about it. Lively is simple, and straightforward, and focuses on doing one thing well: The furnishable 3D chatroom. It can be embedded on a webpage and your avatar can be in multiple rooms at once via different browser windows or tabs.
If you've got a group of up to 20 people (who all have Google accounts and are running Windows), and want to share a Youtube video or sit around and shoot the breeze in a lightweight space, and having your own content isn't for you, then Lively is for you.