Magicplayerzbl

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.

502.9d Assigning damage from a creature with trample considers only the actual toughness of a
blocking creature, not any abilities or effects that might change the final amount of damage
dealt.
Example: A 6/6 green creature with trample is blocked by a 2/2 creature with protection
from green. The attacking creature’s controller must assign at least 2 damage to the
blocker, even though that damage will be prevented by the blocker’s protection ability.
The attacking creature’s controller can then choose to assign the rest of the damage to
the defending player.