There are many types of barrels out there. I prefer the wood ones over the plastic ones. The wood barrels seem to stand up to the weather better and last longer. After you buy the barrel, make sure that it is clean and free of bugs and dirt. Next buy a package of small rocks to put in the bottom of the barrel to prevent the soil from bleeding out. This also helps with drainage. These barrels have a hole in the middle of the bottom. Next buy a 1.5 cubic foot bag of Miracle-Gro potting soil that has peat moss in it. I also recommend adding some mushroom compost to the soil to give the flowers nutrients. Fill the barrel up until you have a 2-3" space from the top of the barrel to the soil. The above steps insure that the flowers will last a long time. You can also add some organic plant food to the soil. This will feed the flowers for at least three months.
Now is the time to go to the nursery to buy the flowers. Depending on the zone you are in, certain plants will do better than others. For example, I have two barrels like the one pictured. I have verbena and petunias in both. I also have some marigolds mixed in as well. I have also planted begonias, impatiens, and asters in these barrels and they have done extremely well. The butterflies love them! Recently I have seen a couple of swallowtails and a monarch.
After you have planted the flowers, be sure to water with at least half a sprinkling can full to promote root growth. Then water every few days so that the flowers get acclimated to the barrel. Since many days of summer seems to be dry, water is very important on keeping your flowers looking great.
Once in a while, you may have to add more plant food or soil as over time it might sink a bit. I also try to weed every few days to keep my barrels looking great.
Every fall, after the flowers have died, I remove the dead flowers and roots and smooth the soil over for the winter. Then in the spring, I empty the soil in the barrels around the trees and put in fresh soil and compost. This gives the new flowers a chance to be just as great if not better than the first batch.