It’s never a guarantee that you’ll go viral. If any agency ever promises this, they’re lying. It’s a combination of luck, time, and good content. Make sure your content is of high quality, if using audio over video that it is synched correctly, include captions/subtitles for videos, don’t copy other users but do adapt trends to be relevant to your own content… there’s so much you can do.
For a dive into TikTok’s virality, we have a blog post on this topic: The Virality of TikTok Content.