When I opened my kit it already had mushrooms in it. What do I do?

This is totally normal. Sounds like you've got a very vigorous kit! If you're growing Oysters, Lion's Mane, Pioppino, or Chestnut you can ignore the mushrooms that may be fruiting inside your bag. If they're easily accessible you can break them off and compost them, so you can start fresh. If you're growing Shiitake, when you remove the bag you should pinch off and compost any of the premature pins.