As mentioned previously, you can begin a slide show with a title slide: in the Slideshow Settings pane, check the Show Title Slide box.
For the title text, iPhoto uses the name of the slide show as it appears in the Library list. But you can change that: just click the text in the slide show editor, then type.
You can also insert subtitle text for other slides: select a slide in the thumbnail browser and then click the Text Slide button. Depending on the theme, the text appears either before the slide or superimposed on it. In all cases, a text slide thumbnail appears to the left of the thumbnail you selected.
You can change ...