Assuming you will be replacing the deck if the deck is applied with screws you would need a drill and an appropriate bit. Most decking screws use a torques bit. If done with nails you will probably need a sledge hammer, an eight pound sledge hammer, a wrecking crowbar and a flat bar. Some nails can be particularly stubborn. It would be helpful if you had a sawzall with a metal cutting blade. Sometimes it's easier to cut the nails off rather than prying off. Harbor Freight sells a good one you could get for about 20 bucks with a coupon.
When installing the new wood you would need a hand held circular saw. I would put the new wood on with decking screws. You would need a drill and the appropriate bit. I would install the wood a little long on the end until you get all of the boards installed and then trim all of them at once. It makes a cleaner edge if cut that way instead of doing them individually. Even offset a sixteenth of an inch really shows up.