Your leading card must be chosen before the monster cards are revealed. Then determine the initiative order by comparing the initiative values on all player leader cards and all monster cards.
From the rules, pg 18 - Determining Initiative
After players have either selected their two action cards or declared a long rest, the players reveal a monster ability card for each type of monster that has at least one gure currently on the map. In addition, each player not taking a long rest reveals his or her selected cards for the round, placing their leading card on top so that its initiative value is visible.
Initiative order is determined by comparing the initiative values on all played monster ability cards and all of the players’ leading cards. Whoever has the lowest initiative value takes their turn first, then the next highest, and so on until every figure on the board has acted.
Remember, when its gets to your turn, the leading card designation is no longer significant.
You can play the top or bottom action of either of your two cards first. Then you play the other action on the other card.