Your Terms must set out who owns or operates the website. Ideally, you will have put the website in the name of a company, to avoid you being caught with personal liability. This needs to be clearly stated.
Users should clearly be told what they can and cannot do with the content on your website. Although you may receive limited protection from background copyright law, it is far more effective to have terms and conditions which clearly state that the content belongs to you and cannot be used by users of your website for any other purpose.
You will also want to warn your users from abusing the site such as by spamming other users, posting defamatory content or attempting to infect the website or app with viruses or malware.
If users are able to post content, then you need to make sure that you have suitable licensing provisions in your Terms to allow you to publish and use that content.
One of the most important clauses in your Terms and Conditions (particularly for those offering business services or sites offering advice or information to users) is to place relevant limitation on your liability, through appropriate disclaimers. Without these in place, you may find that you become liable to unlimited damages claims.
Finally, as the web is global in its nature, it is important to make sure your Terms clearly state that they are governed by English law and the exclusive jurisdiction of the English courts. Otherwise, you may find that you just got served with legal proceedings in the lower court in Outer Mongolia, which is unlikely to be a place you want have to go to defend yourself in person.