I assume that you're going to use vinyl tiles. I tiled my kitchen floor a few years ago, and I used plywood as a base (following advice from the tile makers). That's all that you have to do, and you don't have to put anything else down. Place the nails 6 inches apart along the edges of the boards, and 9 inches apart in the middle. Placing nails at these positions helps to avoid any tendency for the boards to "spring". "... a step into the room": If it's a step up (or a step down) into the bahroom, you can put a carpet edging strip (made of metal, from carpet shops) down, in the doorway, to prevent people catching their toes on the edge of the tiles.