The APA style guide specifically states that italics should be used for:
* Titles of books, journals, and other freestanding works
* Titles of movies, television shows, and other audiovisual media
* Names of ships, aircraft, and spacecraft
* Foreign words and phrases
* Emphasis on a specific word or phrase within a sentence
Underlining is generally discouraged in APA style as it can be difficult to read and is not always supported by different software programs.
If you're unsure whether to italicize something, consult the APA Style Manual or the APA Style website.