So go ahead and roll damage for the first round.
To pin it, you must make a strength roll as per the
grappling rules (except that the opponent doesn't get an opposed attack roll to dodge the grapple). The base chance is 50% +/- 5% for every point your strength score exceeds or falls short of your opponent's strength. The wolf has a strength of 19, thus you have a 40% chance to pin it and hold this round. You can maintain the pin without having to roll your attack again so long as you keep passing your strength check on your turn. If you fail your strength check it escapes and moves freely (of course you can try to pin it again). While pinned with the spear, an opponent can't move, but they can still attack anything inside melee range.