Use the Description box in the Info window.
Or do you mean onto the face of the photo? If so, you'll need an external editor for that.
In order of price here are some suggestions:
The Gimp (free)
Graphic Coverter ($45 approx)
Acorn ($50 approx)
Pixelmator ($50 approx)
Photoshop Elements ($75 approx)
There are many, many other options. Search on MacUpdate. You can set Photoshop (or any image editor) as an external editor in iPhoto. (Preferences -> General -> Edit Photo: Choose from the Drop Down Menu.) This way, when you double click a pic to edit in iPhoto it will open automatically in Photoshop or your Image Editor, and when you save it it's sent back to iPhoto automatically. This is the only way that edits made in another application will be displayed in iPhoto.