Today's short stories must grab the reader on page one. It's even better if they grab the reader in paragraph one, or, ideally, line one. There are myriad ways to grab the reader, some of which I mentioned here last week, including unique voice, engaging character, dramatic problem, etc. The pace of a short story often builds until the end of the story, the climax, and then relaxes in a short denouement.
Today's novels must grab the reader by the end of chapter one. Chapter one usually begins by setting the scene, introducing main character(s), building the world a little. The pace of chapter one usually increases until the end. Thus, ideally, chapter one ends with a dramatic cliffhanger. Obviously, what this cliffhanger is depends on the genre. In a murder mystery, for example, usually it's a body drop.
An effective writerly trick in both is an initial sentence of telling. For example, It was the best of times.... Another effective writerly tool in both is referencing other literary works either explicitly or implicitly. Literature has a long tradition of self-reference.
Whatever you're currently writing: good luck with it!