The amount of insulation you need depends on the climate, type of heating (gas, oil, electricity) you use, and the section of the house that you plan to insulate. The attic is the first area to consider because it is accessible and therefore less expensive to insulate.
A qualified insulation contractor and assist you in determining your homes current R-value.  When considering hiring a professional, be certain to speak to multiple vendors to get a good feel for pricing and expertise.

The next step is to compare the R-value of your insulation with the recommended r-value for your house and your type of space heating. Using these recommended R-values, subtract the R-value of the insulation already in your home. The result will be the R-value you should add.   Many special products have been developed to give higher R-values in a smaller thickness. Some materials require a greater initial thickness to offset eventual settling or to assure that you get the rated R-value under a range of temperature and humidity conditions.

When you stack new insulation on top of existing attic insulation, the existing insulation is compressed a small amount. This will slightly decrease the total R-value of the insulation. This effect is most important if the new insulation is denser than the old insulation. You can compensate for this stacking effect and achieve the desired total R-value by adding about one extra inch of insulation if the old insulation is fiberglass, or about ½ inch if the old insulation is rock wool or cellulose.

For example, consider an existing house in St. Paul, Minnesota with a gas furnace. The recommended R-value for attic floor insulation for this house is R-38. If the existing attic floor insulation has an R-11 insulation value, then an additional R-27 would be needed to bring the attic floor insulation up to the level recommended for that house. Remember to buy the new insulation based on this R-value. Choosing a slightly higher level of insulation, such as R-30, would serve to offset the stacking effect discussed above.