When you want to make a character speak, all you have to do is select it. Then click on the dialog tab.
On top, a status informs you whether the character already speaks a line. At the bottom, options are available for you to add a voice. You can record it yourself, upload an mp3 or select a voice clip you previously uploaded.
If you are in a hurry, you can also use the text-to-speech feature. The voices are not good enough for a professional video, but it can allow you to put a draft video together quickly.