There are several ways.
Look on the bar on the left when you're on your home page.
There's a small group of photos of friends available to chat. Click on one.
Then at the bottom right, there's a horizontal box labled, "Chat" and it will show a number. I think that's the number of your friends availble. Click on the box and it opens. Then you can click on the person's name you want to chat with.
Finally, go to a friends profile.
There's a chat button there, too -- if they're online.