Your question hints that you don't have any scrap wood nor much for tools at your disposal; true?
I would agree with Ron that using cedar or treated wood would work; treated wood is a bit cheaper. Another option would be a plastic/wood composite product. That way you don't need to worry about finish at all. But if you're building a bench for your deck, you may want to use the same kind of wood that was used to build your deck.
As for tools, a handsaw, a carpenter's square or a combination square, and a drill would get the job done. If you're buying a drill, don't bother with the cheapo cordless drills -- you really do get what you pay for, and need to spend a lot to get one that'll work well and last. Also, unless you use a cordless drill regularly, the battery won't last long, and you'll be spending as much for a replacement battery as you did for the drill. Corded drills go for a lot less, and you don't have the battery to fuss over.
One thing to clarify is whether you want the bench to be free standing or built-in; one or the other would affect what size and number of boards you'd need. Also, the room you have available and what you want to accomplish with the bench would determine how big you build it, and that in turn would affect how much wood you'd need.
I've built benches in the past out of deck building scraps; three or four 1 5/8" x 5" deck boards running lengthwise for the seat; two 4"x4" post pieces standing on end at either end for the legs, and deck boards for braces underneath. Very simple.
