You roll to hit first. It is simply a touch attack that is followed by an opposed grapple roll. If you hit with the touch attack, you are then grappling, however, to do anything, you have to make opposed grapple checks. Remember also that if the provoked attack of opportunity lands you do not start the grapple (from the initial touch attack).
Thus, a natural 20 will always hit, causing you to start grappling (unless your hit by the AOO). But Dr. Zoom is right, a natural 20 on your opposed grapple check is not an automatic success.
TLG