The very first thing a reader wants to know is why the things that are happening are happening. Don’t tell him.
In fact, anything the reader wants we shouldn’t give it to him. Make him work for it. The villain hurts our hero and the reader wants the hero to strike back, don’t do it. The reader wants to know why the door is open. Don’t tell him. The reader wants to know why the characters react the way they do. Don’t tell him.
Okay, eventually you have to tell him something, but the longer you make him wait the more he will have to read to figure it out. When you do tell him what’s going on, tell him in such a way that the answer raises more questions. Why is the door open? Because the killer left it open. Who is the killer? Someone who feared the victim. Why would someone fear the victim? And it goes on.