Before you plant your new flower bed, check which hardiness zone you live in (in the United States, the USDA handles this information). Knowing your zone will help you find your first and last frost dates.
Every species has different requirements, but you will normally plant your flowers in early spring, after the last frost. You can also plant many perennials in the fall, before the year’s first frost.
Another thing to remember is that your garden is a long-term investment. Generally, perennials need a year or two to establish. Annuals, on the other hand, will look great from day one.