If one or more helpers are available to chat, you'll see this button at the bottom of the main website:
Don't worry. We are Helpers, not mantises, and we have no intention to eat you. When you click the button you will see a pop-up window to set up your chat profile for the session. The only required information is a name. However, if you include an e-mail, you'll be able to send the chat log back to yourself after it is over.
Edit: To clarify, you're not signing up for anything. It's just a means of identifying yourself to anyone available to answer.
Click Start Chat and you'll see this window, when someone responds, it's chatting time. It operates like a typical IM program. If you made a mistake in the last window, you can also change your name once with the bottom box.
When you are done, say goodbye and click the close button to end the chat.
If there are no helpers online, you can still send an offline message and a staff member will respond when possible. However, an e-mail is required (so that they can respond). Also, you are encouraged to wait until someone is available, which is often.