A good place to begin is the attic. Because heat rises, this is a space where you would notice air leaking the most. Start by plugging up any big gaps and holes. You will most commonly find these in dropped ceiling areas, where the attic meets the floor, and under or behind attic knee walls. Next, you will want to seal up smaller holes. If you are wondering how to detect small air leaks, one indication is that patches of insulation may become darkened so look out for this. Another area of the attic that you will want to check is the attics access panel. For this, you can use a foam board insulation or weatherstripping.
Other than the attic, where else may benefit from being sealed and insulated? Here are a few of them, window or wall mounted air conditioning units, electrical receptacles or outlets, doors, window frames, baseboards, and piping.
Sealing and insulating can help to not only reduce your monthly cooling and heating bills but, when a home is tightly sealed you will also find it improves indoor air quality and comfort and, best of all, it is a job most homeowners can do themselves.