I have just pulled up my old ceramic tiles from my bathroom an there is the thinset on the subfloor an some grout left on it. Do I need to clean that before putting new thinset on it?
Yes, you should remove the thinset. You can usually chisel it up with a hammer and cold chisel.
You don't have to remove it. If it is in good condition and not fragmenting and adhering well, you can apply a layer of fresh thinset over it. However, this will increase the height of the finished floor an extra 1/8 or 1/4 inch (depending on whether you use a 1/4 inch or 1/2 inch, respectively, notched trowel).
If the finished floor should be at the same height as the previous, then get that stuff out. Once you get the knack of getting it to separate from the substrate, it is fairly quick and easy to do. I use an old putty knife and hammer it to slide cleanly under a good edge. Once it is far enough in, large pieces come off.