Flower gardens can be even harder. Start with what colors you like. Rather than basing your dream on a photograph from a magazine, take a look at what your neighbors are growing successfully. They may even be able to give you a division or two.
Take a walk around a couple of garden centers and read the plant labels. Then play with combining the plants that strike your eye until you find a combination of 3-5 plants that pleases you. Make sure all the plants have the same growing requirements (Sun, water, pH...) and that none of them are going to require more care than you can give them.
Keep the variety of plants limited. It makes a better composition to have more plants of less varieties than to have one of this and one of that.