How do i clean a baking stone for pizza properly?

No soap. The stone is porous, it soaks it up and your next pizza tastes of it. Let it cool completely, scrape anything stuck off with a plastic scraper, and wipe with a damp cloth. The dark stains are normal - leave them. Two things that crack stones: putting a cold or damp one into a hot oven, and running it under the tap while it's still hot.