The first place to start is to recalculate your square footage. 8 square feet is only a 2'x4' area. This would not fit a bathtub. If you meant 8 feet x 8 feet, that would be 64 square feet. With a 4" slab, you will need 3/4 yard (19.2 cubic feet) of concrete. About 1 cubic foot/80#bag if you mix yourself (check the labels and get ready for heavy lifting).

You should excavate the area down far enough so that it will not be affected by frost heave. Backfill the excavation with compacted rock to allow for a 4" thick slab to be poured, and put in forms. Be sure to consider drainage direction so it does not pond water against the house. You should probably use a piece of expansion joint against the house. Now, if you want to use rebar or steel mesh, you can lay that in, and you will be ready to pour.