502.9b.

The controller of an attacking creature with trample first assigns damage to the creature(s) blocking it. If all those blocking creatures are assigned lethal damage, any remaining damage is assigned as its controller chooses among those blocking creatures and the player or planeswalker the creature is attacking. When checking for assigned lethal damage, take into account damage already on the creature and damage from other creatures that will be assigned at the same time (see rule 502.9e). The controller need not assign lethal damage to all those blocking creatures but in that case can't assign any damage to the player or planeswalker it's attacking.