Start with the outside of the structure. Make sure that no dirt or debris is piled against the foundations or the walls of the building. If there are branches or other foliage which rest against the walls or foundations, they can serve as a conduit to crawling insects. A termite colony in the walls of your garage can do major structural damage in a short period of time. Residue from wasp nests and even birds nests under the eaves of the garage can discolor the wood and cause structural damage. In addition, the bird droppings look unsightly.
Seal any openings that are unintentional. This can include spaces around windows and doors, broken panes, or openings under the eaves. If the space is to be used as living or entertaining space, you will need to seal not only to keep pests out, but to provide better weatherization. Use thresholds at the entry doors and weather stripping as needed around windows. If you have a garage door opener and want to leave the door open for entertaining or for ease of access, consider using a large screen in the opening. This will provide for light, while preventing much of the influx of flying insects.