I always first try to locate a stud IF its in the appropriate location. Studs are soo good for this even if only one end of the grab bar is attached to one.
Get a drill and get a good tile spade drill bit, they make the work fast and accurate. DO NOT try to drill into ceramic with a conventional drill bit, it will "walk" all over the area and not make a hole. Use the proper tools everywhere.
If you do not have experience or a steady hand, place a piece of masking tape on the hole area and the tape will hold the bit in place also, but you still have to use a tile bit.