Molly, there are many ways to answer this question. First, your plot has to have an aura of potential disaster, danger, or threat. While there is nothing wrong with making that danger concrete, far better to make it a bit vague. That will engage your reader to use his or her own imagination to heighten a sense of danger, of menace.
Secondly, you have to make your reader care about your characters, and absolutely embrace your main character. You want to write in such a way that the reader wants to protect that character, as in, “No, no, don’t go through that door!” But you want to do it without saying those words.
Third: You need to structure your story in such a fashion that the reader is compelled to go on. Here, chapter breaks are a vital tool. Don’t end them by writing, “What was lurking behind the door?” Create action that has the reader ask that question. If you want to create a cliff hanging break, you first have to create a cliff.
Fourth: What academics call foreshadowing—I call it subtly informing your reader—is vital, because it allows the reader to anticipate risks—even if they don’t happen.
Fifth: Selective choice of words is crucial. Don’t fill your text with …”Suddenly, a …” Or, “to her great shock …” If a character is “terrified,” use that word once. Better yet, have the character act in a terrified manner.
Over all, I think the key to suspense is making your reader a partner in the story, getting him/her to perceive potential misadventures. Think of it this way: Your primary task is to fill your reader’s mind with the anticipation of alarming possibilities. If you do not do that, you will ….