Outline your story in chronological order so you know what you need to have happen from beginning to end.
Read through the outline and choose the most interesting point in it. Start your story there to immediately draw the reader into the story. The stuff that happens earlier can be told later as back story or flashbacks.
Write an opening and an end sentence for that section you're focusing on that will make the reader ask questions. For example, the sentence "John died twenty minutes ago" makes a reader ask who John is and why or how he died. Those unanswered questions are the key to an effective hook.
Ask friends to read your hook without any other information about the story and see if they are interested and want to know more. If they do, the hook is doing its job. If not, edit it some more and build more suspense or mystery into it.