how to create a page that requires you to accept the general conditions of the forum?
I think that would be a function of a membership plugin like Ultimate Member.
You could also use Gravity Forms with the User Registration addon to get users registered, with a field that asks them to agree to the terms. After the GF form is submitted, you could redirect them to a page with further instructions about filling in their profile, etc.
There are many ways you could address this, but these are a couple of suggestions.
wpForo 1,4,0 comes with wpForo User Custom Fields addon.