おじょうさん has fairly limited use. The first meaning glosses pretty closely as "Miss" in English, meaning unmarried woman, and would sound a little strange coming from a young man (much like calling someone "Miss" does for young men in most English dialects). You also have to be careful with it because it's used colloquially to mean spoiled or, more literally, "princess-y" (and, because you're younger and someone of your age would rarely use it, it's potentially more likely to be interpreted that way). おねえさん can be used, but more as a term of reference than a term of address. Ex. となりのおねえさんのねこがはしゃいでいる　= the neighbor lady's cat is bouncing all over the place. Here you're talking about her but not to her. Unless you're speaking humorously most young people won't おねえさん as a term of address (unless maybe you're trying to get her attention in a crowd because she dropped something). Think of it as meaning "younger woman to whom I'm referring but whose name I don't know". Hopefully others can chime in to clarify some points, but I think this should get you on the right track.