Even a strong wall shelf will only be able to bear a little weight if it's fixed to the wall with the wrong fittings. For a wall shelf to be practical and secure you must use the right fittings for the wall type. Check if your walls are concrete, plaster or wood. There are fittings for all wall types. If you can, try to determine what part of the wall is load-bearing. The most secure place to mount a shelf is on the bearing part of the wall.