I was looking thru the book for rules on how resisting Psychic powers actually works but couldn't find anything. When someone is hit with a Psychic power do they always get a chance to resist it with a Willpower roll? Is it opposed at all? Do they only get a roll if they have the talent?

It depends on the power. Some Psychic powers, especially some that affect the mind, have rolls to resist. Some more combat related ones, like Force Bolt, have no resistance roll. Whenever you get to resist a roll, Resistant (Psychic) gives you a +10 to the roll. It does not allow you to resist things you could not before.

As far as I know, if a psychic power does not allow for an (un-)opposed WP test in its description it cannot be resisted. Thus a power such as Spasm can be resisted whereas something like Unatural Aim or Fearful Aura (which enhance the Psyker) cannot.