You may need a Terms of Service but also a Privacy Policy, which is required by law if you collect personal data.
The Privacy Policy is also required by Mixpanel. If you're using the tracking tool it means that you're agreeing with the terms of Mixpanel (https://mixpanel.com/terms/). The terms require you to inform users that you are using Mixpanel. You also need to inform users about the opt-out policy of Mixpanel.
>>> include a notice about the Mixpanel opt-out for Customer’s site and the Mixpanel opt-out link in Customer’s privacy policy or in a notice on Customer’s website(s).
>>> Terms of Use of Mixpanel
The Terms of Service agreement isn't usually required by law but you can use it to include guidelines and rules for users to agree with and follow if they want to join your community (i.e. no spamming other users, no abusive accounts etc.)
Hopefully our two presentations on these agreements will help you:
1. What's a Privacy Policy --> https://www.slideshare.net/termsfeed/the-privacy-policy-agre...
2. What's a Terms & Conditions --> https://www.slideshare.net/termsfeed/whats-a-terms-and-condi...
Also, it's important to research how to integrate and get consent to the legal agreements you make available to your users. For this, research "clickwrap".