How To Prepare Floor Before Laying Tile?

You don't say what type of floor you are tiling, but one of the most important things is to make sure the floor is in good shape, level and clean from any dust or debris.
If the floor is not level, you can purchase a self leveling compound from most home supply stores.
  If your floor is plywood or wood, suggest you skim it with a leveling compound and let it cure for about 48 hours before tiling. This will seal the floor and create a good bond for the thin-set mortar mix you will be using to attach the tile.
If you don't want to skim the wood flooring, then you can screw down backer board over the existing floor, but be sure to check the clearance of the door, (if there is one).