Why is calcium fluoride insoluble in water?
This answer to a question about the solubility of silver nitrate and silver chloride will give you a good starting point to answer your question.

In order to dissolve a salt, you have to break apart the ions and hydrate them in solution. You can use the enthalpies of hydration of the ions, and the crystal lattice energy of the solid, to predict which compounds will dissolve. There is quite a nice discussion about how to think about solubilities of salts, and what factors affect solubility using a Born-Haber cycle, in this reference. The values for enthalpies of hydration and crystal lattice energies of sodium chloride and calcium fluoride can be found in standard chemistry texts or with an on-line search.

If you have more questions after looking at this material, come back and ask for clarification.