You would only say "in the stairs" if something was physically inside of the staircase — quite rare. Otherwise, it would be "on the stairs." I am standing on the stairs, we are sitting on the stairs, the suitcase is on the stairs, etc.
The styles are slightly different, but it would be a "tank top" (occasionally just "tank") or a "sleeveless tee". Do a Google Image Search to see the difference.