Hi Paul - Go to HD's website, find "construction materials," then "cement and concrete." You will find rebar, tie wire, etc. I can't tell the size of your project from the picture but 10' lengths are easier to handle, and you can get some 6' and 3' pre-cut lengths for cross pieces. Lay out a grid with 1-foot squares and calculate how much of what lengths you will need. Do it on paper first. There should be a perimeter of rebar about 6" in from edge of slab. Support the grid on little concrete blocks about every 3 feet, to keep the rebar in the center of the slab when the concrete is poured over it. You can grab it and lift it in the wet cement to be sure it is more or less centered in the slab.
You can rough screed the surface with a 2x4 but there are tools made for this purpose that are easier to handle. You should also get a steel float, essentially a metal spatula, to sweep across the surface to produce a smooth finish and press the aggregate down just below the surface. There are hand-held ones in metal and wood but the long one (kind of like a pool sweep) is easier to use and can reach the center of the slab easier than one without a long pole.
You are going to need 3 or 4 people to shovel, move the wet mix, do the leveling. They will need rubber boots and rubber gloves, concrete is really nasty on bare skin. You also may find a local guy who does this kind of work by asking the concrete supplier. It would be worth a few dollars to have someone on site that has done this before and has some expertise. This structure is going to last 200 years so no sense scrimping now.
Good luck with the project!. There are loads of Youtube videos on how to do concrete and rebar placement, have a look.