Firstly I would like to say that Google chat isn't very popular
: many people don't use it even though it can be used directly from your Gmail ID.
Secondly, you can use Gmail Chat in two ways:
Either you download a small piece of software called "GTALK" (search for it) and run it on your computer.
Or you can chat directly from your web browser while viewing your Gmail Inbox. (At the bottom there is an option for enabling or disabling chat)
Feel free to ask again !