Check your local nursery for good ideas in flowering annuals. There are some really nice blue and purple salvias you might like, and lots of pinks to choose from, and don't be afraid to combine low plants and high ones, like a couple of petunias and a salvia.
The main thing is to fill the containers with a top quality container soil with added fertilizer. Miracle-gro gives great results. Just filling the pots with a shovelful from your back yard will work, but not as well.
Make sure that the pots have drainage (a hole in the bottom that isn't clogged up) and then be consistant in watering them. Containers set in the sun on concrete can get boiling hot and dry out in no time.
Enjoy!