I have been telling stories for many, many years and worked with some of the most awesome storytellers out there. When I look for a story for adults, and I do usually tell for adults, I choose a story I like and I have an affinity with. Then I look at the language I use, the phraseology, vocabulary, internation etc. I always start with a 'safe place', it could be setting the story in a locality known to all listeners or in a setting which is close to you, but I never start with 'this is a story about..' I always start as if I am about to have a conversation with someone, and you are, except they don't get to talk to you! Eye contact is very important, it makes people think you are telling the story directly to them.. and you are. Telling a story is like singing a song, rehearse it, not until you get it right, but until you can't get it wrong. Remember the bones of the story and weave your magic around them. You'll never tell exactly the same story twice. But whatever you do you must bring your listeners 'safely home'. You cannot leave them hanging in mid air. A comfy start and a safe ending. As for length, I can tell a story, and I do tell the traditional stories of Cornwall, that lasts 3 minutes or I can do a 45 minute epic, although I wouldn't do that at a folk club. As for sources.. find things you like, that suit you and that you are comfortable with. If you have that and you choose your vocabulary well, you modulate your voice well and pull people into the story you could tell them goldilocks or red riding hood and it would work. Most importantly... don't stop telling or keeping them to a storytelling club. There's a huge audience out there.