The wall above the new door will have to be strengthened by putting in a header. This is because the outside walls of your house are load bearing, and if you remove some of the studs to create a door opening, the load of the roof from above needs to be carried around each side.
The deck would typically be built by fastening a ledger board to the wall of the house so support one side of it, and the outside edge of the deck would be supported by two posts that sit on two concrete footings. The ledger board needs to be fastened into the wood structure of the house.Both projects involve structural planning, would need permitting and be given drawing approval by your local municipal center.