Attacking is the action your creatures in the battle zone can take against either your opponent or his tapped creatures in the battle zone. To have one of your creatures attack, tap it and declare what you want to attack against. There is no limit to the amount of times a creature can attack each turn as long as it is untapped and you can tap it.
When a creature attacks another creature, they battle. The creature with the higher power wins. The losing creature goes to its owner's graveyard. If the powers are tied, both creatures go to their owners' graveyards.
When a creature attacks your opponent and isn't blocked, you choose one of your opponent's shields. (Creatures that have "This creature can't attack players" can't attack shields.) Your opponent must put the broken shield into their hand and puts any fortresses fortifying that shield into his or her graveyard. If a creature attacks your opponent when he has no shields left and isn't blocked, you win the game.