Yes on formating the sd card. I do it everytime i put a card in . it erases all the pictures on card
Yes on reuseing the card. like said above formating erases them.
I carry a card reader with me, set the camera up and walk in front of it to make sure its the height that will get the animal your after. that way you won't be disappointed when you come back.
also make sure no tree limbs that will set it off.
good luck