There are several ways to communicate with other avatars in Second Life.

The first is a general chat. Just click on the blue button on the bottom of the screen labeled Chat.
Then, in the open white space can just type in "Hello" or "S'up" and avatars around you will see your greeting. Your avatar will show a typing motion when he or she is communicating in the chat-mode.

Alternatively, you could just click on the blue button on the bottom of the screen labeled IM. This will allow you to choose an individual avatar with whom you can speak. Just click on the avatar's name to begin a discussion. The name will appear on the bottom as a tab. Make sure the right tab is selected when you begin a discussion. (note: no typing motion appears in chat mode.)

If you see an avatar and would like to begin a IM, right-click on the avatar and select the option that appears called Send IM. Then you can begin a discussion.

One way that objects can "speak" to avatars or convey information is through a notecard. A notecard contains information that you might find valuable in terms of using an object or describing a particular area. Notecards are a great way for providing educational information. You can obtain a notecard by opening an item or just by clicking on an item. This note card was obtained from the cube on the right.

Here is a tutorial on how to create and send a notecard.