What is an important structural molecule of cell membrane ... ?

Most important and abundant one are the phospholipids - amphiphatic molecules with an hydrophilic head and a hydrophobic hydrocarbon tails. They compose the membrane bilayer. Also important are the membrane proteins, which can be periheric and internal and can serve as channels, receptors, pumps etc.

Depending on the cell type the membrane can also contain cholesterol (in animal cells, enhances the membrane stability) or hopanes (in prokaryotes, same function as cholesterol) or ether-phospholipids with branched hydroharbon tails (in archaea, make the membranes more stable and resistant to severe conditions, as archaeas are extremophiles)