Mayhem represents an alternative cost that can be paid to cast a spell from your graveyard if you discarded the card with mayhem that turn. You must pay any additional costs the spell has.

If a card in a graveyard has {X} in its mana cost, X is 0 when determining its mana value.

You must follow all normal timing rules when casting a spell with mayhem. For example, you can't cast a creature card with mayhem you couldn't normally cast during an opponent's turn, even if you discard that card during an opponent's turn.

If the returned creature can't attack for any reason (such as being tapped or having come under your control that turn), then it doesn't attack. If there's a cost associated with having it attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.