Use anchor bolts. Since your slab already exists, you would have to drill holes, and epoxy the bolts in the holes (vac the debris out first). Space the bolts, so they don't fall on a stud.

If you could, but this would be more work - lay a course of block. This will keep the sill plate off the ground (won't rot) and keep out the water. Use pressure treated for the bottom plate.

If this is a pre-cut shed, and you raise it off the ground, you will have to adjust the door height.

I would pour an additional 3" of concrete on top of your existing slab, which is about 23 cubic feet or just under 1 cubic yard. Place steel mesh sheets and anchor bolts for your sill plates in wet concrete.

Raising the sill plate with a 8" course of CMU would be my next best option, but may allow water to penetrate the inside, depending on grading.

geoffrey_b If I lay the block down first do I just use mortar to adhere it to the concrete?

Yes, use mortar.