Bees are losing habitat all around the world due to intensive monoculture-based farming practices (large farms of one crop), pristine green (but flower-barren) sprawling suburban lawns and from the destruction of native landscapes. Just planting flowers in your garden, yard, or in a planter will help provide bees with forage. Avoid chemically treating your flowers as chemicals can leach into pollen and negatively affect the bees systems. Plant plenty of the same type of bloom together, bees like volume of forage (a sq. yard is a good estimate).
Here are a few examples of good plant varieties: Spring – lilacs, penstemon, lavender, sage, verbena, and wisteria. Summer – Mint, cosmos, squash, tomatoes, pumpkins, sunflowers, oregano, rosemary, poppies, black-eyed Susan, passion flower vine, honeysuckle. Fall – Fuschia, mint, bush sunflower, sage, verbena, toadflax.