Open iChat by clicking its icon in the dock.
If iChat doesn't automatically sign you in click Log In to ... from the iChat Menu.
Now click to select Preferences... from the iChat Menu.
Select the Messages tab from the Preferences Window.
Click to check the box entitled Collect chats into a single window. You can now close the Preferences window.
Double click a buddy from your contacts list to open a new iChat window. Notice we still don't see tabs on the side.
Double click another buddy from the contacts list. Now notice that a side panel slides out of your iChat window and allows you to manage both conversations from a single window.
You can easily close a conversation by moving your mouse over one of the buddies in your side panel (tabs) and clicking the X button that appears.