I think a good start is looking in the challenge gameplay forum. There are so many stories where you can get inspirations for you own story. You don't need to play the legacy for it. I think at the beginning there is a lot of trial and error, especially in the game itself.
For me the game should relax me and do not do the opposite. In one game I tried the rebuilding the world challenge with 5 couples, while it was fun at first, I quitted when all families had 4 or more toddlers, one family had 5 daughters, but the heir had to be male. The Story was in my head, but never again, I swear. On the other side, I can't see my sims dieing, when I put so much time in them.
ArcherDK has explained it very well.