How do Van der Waals forces affect the boiling point?
The stronger the Van der Waals forces, the higher the boiling point.
To melt or boil a simple molecular substance, we only have to overcome the intermolecular forces. Looked at the other way round, if it weren't for intermolecular forces, everything molecular would be a gas. It is the intermolecular forces acting between the molecules that cause attractions between them making them liquids or solids.
So, the stronger the intermolecular forces are, the more energy will be required to overcome these in order to go from the liquid to the gaseous state, and hence the higher the boiling point will be.
The strength of Van der Waals forces depends primarily on the number of electrons in total in the molecule, so larger molecules will have higher boiling points. For isoelectonic molecules (isomers with the same number of electrons) what determines the strength of the Van der Waals forces is the area of surface contact. The more branched the isomer is, the less closely they can pack, and hence the weaker the Van der Waals forces will be.