Perform a tour of your home to determine where it may need insulation. A good rule of thumb is that all heated or cooled areas should be separated from un-conditioned areas with insulation. Regardless of your home's layout, you can use this rule to determine where insulation should be installed. Each area will have its own priority in terms of insulation ease and cost-effectiveness and should be evaluated on the basis of both.
To know if your home currently has enough insulation, contact your local contractor to find out the recommended insulation levels for various parts of a home in your area. They will be able to tell you if your home meets the recommended R-Values for ceilings, walls and floors.
In order of cost-effectiveness for an existing home, it generally pays to insulate first your attic or roof, second your foundation or floor, third your windows, and last, your walls. If you are unsure of where to begin, you may want to have an energy audit.