Lipids have a hydrophobic part and a hydrophilic part. explain how these properties affect their ability to forma bilayer.?

This might make more sense if you read through it with a simple drawing of the phospholipid bilayer to refer to!
As there is fairly high water content both inside and outside of cells, the hydrophobic part of the lipid doesn't want to be in contact with the inside OR the outside of a cell. In contrast, the hydrophilic part of the lipid is happy to be in contact with both inside or outside of the cell. To prevent the hydrophobic parts from being exposed to either side, a bilayer must form. This is because a single layer would mean the hydrophobic part would be forced to be in contact with water. 
As the hydrophilic part of the lipid is polar and water is also polar, they will be attracted to each other. Likewise the non-polar hydrophobic part will be attracted to other non-polar molecules. So in the bilayer, the hydrophobic part of the lipids will be in contact with each other and not in contact with either side of the cell (which works according to the rules of attraction above) while the hydrophilic parts of the lipids will be exposed inside and outside of the cell, as they are attracted to the water molecules.