It is mostly important to know what climate zone you live in. That is very important on what plants will grow in your area. You also need to know how much sun or shade you have in order to plant the correct plant for that location. Make sure your soil is well drained. Once you plant your flowers, make sure you use mulch--it will save you on weeding and will keep the flowers cooler in the summer and protected in the winter and retains more water for the plants. Also make sure you use an all purpose fertilizer once a week or twice a month to give the flowers the nutrients they need. Make sure that once the flower blooms then dies that you cut off the flower so that it will continue to grow more flowers. It also depends if you want to grow annuals that will bloom just for the season, or a perennial that will come back year after year. Annuals that are easy are petunias, geraniums, marigolds.
perennials that are easy are shast daisy, coneflower, black eyed susans, lilys, phlox. Good luck and go to Michigan Bulb.com to get more ideas as to the different flowers you can buy.