Relationship between Van der Waals forces and the boiling point
We talk about the forces between molecules.  The Van der Waals force represents the summation of all attractive and repulsive forces between molecules, other than those due to electrostatic charges or actual bonds.  This summation most often turns out to be a weak attractive force.  Typically the magnitude of the Van der Waals force is small compared to electrostatic or bonding forces and (again typically) only apply at relatively short range.  So they usually serve to weakly hold molecules together.  The stronger the Van der Waals force, the more strongly molecules will be held together and the more energy will be needed to be put into the system to break them apart from each other.  In the case of boiling a liquid, you are really talking about breaking molecules apart from one another.  So in the case of boiling, the weaker the Van der Waals attraction between molecules, less energy will needed to break them apart from each other and allow them to boil.
Here is a link to a Wikipedia article on Van der Waals forces if you'd like to read more about them.