Boxes can be bought ready-made or you can build your own. Just make sure it's securely fastened to your house. Add enough compost and good potting soil around the plants so they sit firmly.
Window boxes look their best if they're crowded with plants.
Start early in the spring with tulips or other blooming bulbs. Later, replace with summer flowers. Some great choices are geraniums, lavender, sweet alyssum, and ivy. Or you might want to plant herbs like sage, chives, thyme, and mint. Make sure to water and fertilize often.