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Look for cracks in the walls, windows, and doors. Walk around the outside of your home. Ants can enter in small areas, so fill as many as you can find.  Use a silcone-based caulk meant for the outdoors. It must hold up against the weather. Also, don't forget you'll need a caulking gun. Don't cut the tube too far down. You need to cut it low enough to release enough caulk but not so far that it globs all over the place. Apply even pressure, and fill in the gap with caulk. With a glove on, smooth the caulk down. You need to remove the smell that ants leave behind, as it is the trail they follow. Don't forget to spray under the doors. Go around the outside of the house, spraying insecticide as you go. Don't forget to spray around the windows, as well.
Take a tour of your home. Fill any cracks with caulk. Clean the borders of doors with a detergent. Spray an insecticide around any cracks. Spray the outside of the house.