Put any number of target creature cards from your graveyard on top of your library.
Draw a card.
If you choose any target creature cards and they are all illegal targets by the time Forever Young tries to resolve, the spell won't resolve. You won't draw a card.
You choose the order that the creature cards are put on top of your library. You don't have to reveal this order to other players. Whichever ends up on top will be the card you draw.
You can cast Forever Young with no targets if you just want to draw a card.
