30x10 porch --> small appartment/room
If you're asking whether the framed wall should rest on the concrete, the joists, or the subfloor--the answer is the subfloor. This is how platform framing works. Joists, subfloor, sole plate, in that order (from bottom to top).
That said, I'm seeing a possible train wreck here. I'm afraid to ask how you plan on dealing with the slanted slab... Do you have a detailed plan for how everything is going to come together, or are you just going to figure it out as you go?