Creatures are your primary means for dealing damage to your enemy. A creature can be summoned to the battlefield by tapping the required amount of mana.
Creatures are able to attack the opponent, or any planeswalkers that the opponent has on the battlefield, and creatures can block attacks from the opponent's creatures. Creatures are not permitted to target the opponent's creatures.
A creature that was just summoned cannot attack until the next turn, unless it has haste, which allows it to attack right away. A creature can block attacks right after it has been summoned.
A creature has two numbers in the bottom-right of the card: power, and toughness (respectively). Power is how much damage the creature deals when it attacks. Toughness is how much damage the creature can sustain before being defeated.
When an enemy creature blocks your attack, the blocking creature will deal damage to your creature equal to its power. In addition, your creature will deal damage to the blocker equal to your creature's power.