The various symbols are usually as follows:

Earth Rich loam in an earthenware cup, inscribed with a
      pentacle in the bottom.

Air  Incense placed in an earthenware cup and burned
     during the rite. Vegetable oil in a earthenware cup.
 
Fire  wick which is lit during the rite. Or a candle
      placed in a earthenware cup.

Water Pure water in an earthenware cup.

The direction of the Altar is considered important. However, it varies
depending on your hemisphere. The Earth and Fire are reversed, and some consider reversing Air and Water, although this is not necessary.

         Element   Northern Hemisphere    Southern Hemisphere
           Earth             North                       South
           Air               East                        East
           Fire              South                       North
           Water             West                        West