1. I think you are referring to a step-stool. A stool is a simple type of chair. A step-stool is a stool with a step that can be used as a tiny ladder.
2. In this context, “story” is a synonym for a level of a building. In British English, I believe it’s spelled “storey”. The plural is “stories” in either case, but we don’t pluralize measure-words used after a number as a compound adjective. For example, “Joe went for a four-mile run.” We say “mile”, not “miles”. The meaning is the same as the longer, less common phrase “a run of four miles”.
Note: I’ve been reading your questions for a long time now, and you would benefit *greatly* by improving your pronunciation, because the way you speak is the way you expect to hear. You always have the same issues — confusing final L and R, hearing the wrong vowels, for example. By the way, it’s impossible for an English word to start in SD-, SB-, or SG-. However, if your native language is Mandarin or Cantonese, English T, P, or K will *sound* like D, B, or G after an S sound (because most English speakers do not aspirate consonants in that position).