Click on the Title icon on the Toolbar and drag a title onto a clip in your Project Timeline.
The title will appear as a blue box above your clip which can be dragged anywhere along the timeline.
To change the duration of a title click and drag one of its edges in/out.
To edit a title, select it and then the text can be changed in the Viewer. click on "Show Fonts" in the top left of the Viewer window to alter size, font, colour, etc (for more control click on "System Font Panel")
To change the type of title, double-click on it in the Timeline to open the Inspector. Then select "Title" to view other styles (some have limited editing options).
To copy a title hold alt and drag it over to another clip
Titles can also be added to a plain background. Rather than dragging them over a clip, simply drag and drop them straight into your Timeline.