Lego Universe (LU) allows you to define behaviors to go along with the models you find while playing or construct yourself from bricks. Once you've cleared your first property (Block Yard in Avant Gardens), you can start building on it and adding simple behaviors to your models. This short tutorial is designed to get someone new to building in LU comfortable with the behaviors system. It shows how to create a simple custom behavior on a model.
Let's make a behavior that welcomes visitors to your property. Note the highlighted area below; you must switch into behaviors mode (#2) to edit the behavior of this model, instead of lifting it up or putting it away.

Once you're in behaviors mode, press Shift. This activates behavior editing of the model. Note how the screen shows a square with a plus sign and a circle with a pencil. You can drag pre-created behaviors from your backpack onto the square or create custom behaviors with the pencil. Let's do the latter.

Once you click the pencil, you are editing custom behaviors. The dialog that comes up shows you a large black workspace for the assembled behaviors plus a row of action groupings below. There are several pages of these actions, but the workspace is shared between them. Now, let's pick one of the yellow start action blocks and drag it onto the workspace. Here I've chosen the third one, Near, which triggers when a minifigure is near the model. (Note that the exact behavior is adjustable, but let's leave the default for now.)

Now we want something interesting to be associated with the Near start block we created. Let's switch over to another set of action blocks by clicking the explosion icon (fifth set of actions). Let's drag a Chat action onto the workspace, joining it to our Near action. Initially, the chat says "Hi", but let's change that. Click in the white rectangle and edit the text. (Make sure to give it time to validate the text and turn green before you hit return.)

That's it for the behavior itself. It's convenient to give our new behavior a name as we save it, since we can just mouse over the icon and get an idea of what it is without opening it. (You can also drag these named behaviors to your backpack to use with multiple models.) Click the top center of the dialog and type the name for your behavior. Then click the check mark just to the right of the text entry. (This text must also be validated.)

Finally, we're ready to save our work. Click the larger check box at the upper right corner of the dialog to save and close.

You now have a behavior on your model! Take off your thinking hat, back away from the model and move towards it, and voila!

You can go back and edit this behavior as many times as you want to get it working properly. Use the pencil in your new behavior to get back into the dialog with the workspace and palette of action blocks.
I have also created a YouTube video of the whole creation sequence.