Select the first one you wish to add a caption to and press Command-I. This produces the Info pane on the right side of the iPhoto window. Click where it reads Add a description… and do exactly as it asks—enter your caption. Repeat this process for each image you want to eventually print.
For More Information Please Refer: https://www.macworld.com/article/2092344/how-to-print-captions-with-your-iphoto-images.html