Recipes and preferences differ as to whether an octopus should be boiled whole or first cut into pieces. They can differ also regarding whether or not to cut off the tentacles. In most cases these simply are a matter of individual preference.
One agreed step, however, is to cut the middle piece and the head. Separate the head from the middle piece and cut the head in half. If not pre-cleaned, remove and discard the beak, innards and the ink sac. These are visible once the head has been halved.
Ask the fishermen — they will gladly clean your seafood purchases for you.