Definitely look carefully at the absolute worst that can be found on the Internet, and the worst that sites with user-submitted content have to deal with, and decide in advance what you do and don't want to support. Anything you don't want to say "yeah, I'd be completely fine hosting that without hesitation" about, you should think about now, and put in corresponding policies now.

Having policies in advance means people will just go elsewhere. Trying to add such policies later, when you have existing users, will be much harder. You're likely to want to draw that line somewhere; better to draw it in advance.

(I've dealt with user-generated content before; please feel free to reach out privately if you'd like to talk about it further.)