Planting flowers in containers is really easy you'll just want to keep a few points in mind.
When deciding on flowers for your container most people go with the thriller/filler/spiller method. The thriller is something tall for the center of the container like ornamental grasses. The filler is a bushy type of plant that will fill in all the holes but not overwhelm anything. The spiller is a cascading type, something that will grow out of the containers and spill over the sides.
When choosing plants you want to consider their eventual size not the size at planting. Leave room for them to grow into their new habitat or they become root bound and their growth will be stunted.