The chat log is located in the bottom left-hand corner of the screen. In this you will see chat from all the channels you are currently in. To talk you can press enter or type '/' and a field below the chat box will pop up for you to type in. To talk in specific channels you will need to type '/channelname' and then space and then your message. For example you could type '/say hello' and this will show up as '<Your name> says hello'
There are also public channels which will be numbered and to type in these channels you can type '/#' with the number the channel is displayed as. You can also leave these channels by type '/leave #' or '/leave <channelname>'. To rejoin these channels you will need to have the actual channel name though and join by typing '/join channelname'.
All these options are also available by right-clicking on the general tab at the top of the chat log and clicking on settings. Here you can also change the font size and color of the chat.