A good starting point is the attic space as this is the area where you would most likely notice leaking air. Begin with plugging up any bigger holes and gaps, these are commonly found in dropped ceiling areas, behind or under attic knee walls, and where the attic meets with the floor. Any smaller holes you find can be sealed. Small air leaks are particularly noticeable in insulation because they cause it to darken so look for areas like this. Another area that you should check is the attics access panel, this may benefit from foam board insulation or weather-stripping.
As well as the attic space, what other areas of the home would benefit from sealing or insulating? Among the many areas are piping, electrical outlets or receptacles, window frames, doors, baseboards, and window or wall mounted air conditioning units.
Taking the time to go through your home and look for areas in need of insulating or sealing will not only help to lower your monthly cooling and heating bills but, a home that is tightly sealed will also contribute to improved indoor air quality and comfort.