Installing a towel bar or toilet-paper holder often requires drilling through glazed ceramic tile, which is no easy task. Use a carbide-tipped masonry bit; regular twist-drill bits can't cut through the fire-hardened glaze. You'll also need to keep the bit from slipping off the glossy surface.
Begin by making an "X" over the tile in the spot to be drilled using masking tape. Be sure to measure again after applying the masking tape so the hole is not misplaced, then start your drill bit into the tape. The tape gives the bit the necessary surface traction.
AsktheBuilder.com: Drilling ceramic tile requires a special drill bit and a steady hand. A carbide drill bit or a diamond drill bit are best for drilling holes in ceramic