You can level the floor with the tile mortar or "floorstone".
If the floor is less than 1/4" out-of-level, you can build up the mortar as you are applying the tile using a larger notched trowel in the low areas and the recommended notched trowel in the higher areas. The recommended notched trowel size is dependent upon the size tiles you are laying. This method may require more skill than the method below.
OR
You can also use a "floorstone" leveling mix which is sort of a light colored, soupy, cement mix. It is most often used under vinyl tile and carpet. Most home building suppliers carry it under various names. Some brands are "self-leveling" but don't always work as advertised. I find that I usually have to trowel them just before they set up. Don't place more than you need. It is a lot easier to add another thin layer than to grind away an overage. Make sure that the product is recommended over your wood substrate. After it dries, you can install the ceramic tile in the normal fashion.