Yep, you need to nail down cement backer board before you tile it. Have you ever tiled before? I've done 2 tiling projects and I found it helpful to go to a tiling class before I got started. I bought our tile from a giant tile superstore that help free classes and I've seen free classes at the large home improvement store we go to. There are some things that you'll learn that you probably wouldn't think of without the training class. Good luck!