If a creature attacks a creature, the controller of the attacked creature is the defending player. That player’s creatures that aren’t attacked can block the attacking creature.

A player can use an attacked creature to block a creature attacking it or choose to not block with it at all.

An unblocked attacking creature that’s attacking a creature deals its combat damage to the creature it attacks and vice versa, following the same rules for an attacking creature and a creature blocking it. However, if the attacking creature has trample, it can’t assign trample damage to the defending player.