Research on TikTok has grown along with the app's rapidly rising global popularity. In this systematic review, we investigate 58 articles examining TikTok, its users, and its content. Focusing on articles published in journals and proceedings across the domains of human-computer interaction, communication, and other related disciplines, we analyze the methods being used to study TikTok, as well as ethical considerations. Based on our analysis, we found that research on TikTok tends to use content analysis as their primary method and mainly focus on user behavior and culture, effects of use, the platform's policies and governance, and very few articles discuss the ethical implications of collecting and analyzing such data. Additionally, most studies employ traditional forms of data collection when the affordances of TikTok tend to differ from other social media platforms. We conclude with a discussion about possible future directions and contribute to ongoing conversations about ethics and social media data.