Communication with teenagers who are significantly affected by sexually transmitted infections (STIs) is essential for the sake of prevention. The aim of this study is to develop a specific questionnaire for surveying the degree of knowledge, behavior, and attitudes of current teenagers and young adults on STIs to come up with the proper training tools. We conducted the study following the Delphi method, a 2-round critical assessment score (from 1 to 9) of all domains and items. Only domains and items with median scores ≥8 were selected. A total of 8 panelists were involved in this survey. After establishing a median score ≥8, a total of 14 domains and 40 items were eventually selected. This is the first questionnaire ever conducted to study the knowledge, habits, and attitudes of contemporary teenagers and young adults on STIs, and stands as a valuable tool for future training on STI prevention in teenagers and young adults.