Example:
Articulate the subject you want to investigate:
How does religiousness impact the sexual behavior of college students?
Identify the words/phrases that constitute the main topics:
How does religiousness impact the sexual behavior of college students?
Start searching using your own vocabulary for your topics
Review some of the full records on the result list and look for PsycINFO subject terms that are synonymous with your search terms
Redo your search combining all the database subject terms you found:
religiosity or "religious beliefs" or "religious affiliation" or "religious practices"
AND
"psychosexual behavior" or "sexual attitudes" or "sexual risk taking" or "premarital intercourse"
AND
"college students"