How do i cancel a giveaway before starts?

exactly like this
someone wins the game, you give him the key/gift/redeemable-link/whatever.

Here you making giveaways for games that you either have an unused steam key or a steam copy of the game in your inventory etc. (not steam library). When the giveaway ends a winner will be randomly selected and you have 7 days to sent the gift to the winner.
Take the time and read the FAQ for better understanding.