Riddleform's first ability resolves before the spell that caused it to trigger, but after targets for that spell are chosen. It will resolve even if that spell is countered. This means that if a spell affects each creature or each creature you control, it will affect Riddleform if you choose for Riddleform to become a creature, but a spell can't target Riddleform if that spell requires a "target creature."

If Riddleform becomes a creature the same turn it enters the battlefield, you can't attack with it or use any of its abilities (if it gains any).