Building a successful container garden is no more complicated that choosing a container, filling it with media and planting the plants. Container gardens tend to need a bit more attention than a similar garden in the ground. You need to pay close attention to the potting media you use, how you water and fertilize and the types of plants you choose based on location of the garden. All of these will determine the success of the container.
First, fill the container with media. The section on selecting a soil media outlines the types of media available and what you need to consider based upon what you will be growing. In any case, all medias need to be well drained yet retain enough moisture for good plant growth. Bagged media have ingredient labels on them. If they are unclear, ask your retailer to explain what they mean and maybe even ask what type of media they prefer to use for their containers.
Bagged medias are easier to use when they are slightly moist prior to filling the container. Pre-moisten the media and then fill the container to within about an inch of the top of the pot. This space is referred to as head space and provides a place to put water and have it soak down into the container and not just run off the top.
What type of plants you choose will depend on location -- sun or shade. How you arrange them depends on how you will be looking at the pot. If you will be looking at the pot from all sides the classic approach is to place tall plants in the center and then surround them with shorter mounding plants and finally finishing off with trailing plants along the edges. If it will be viewed from only the front locate the tall plants at the back and work forward with shorter plants and then trailing types.
If you are not sure of your design, set the plants on top of the media to visualize the arrangement before you start to plant. You can then adjust as needed. Once you have settled on a design, start planting. Remove the plants from their pots and look at the roots. If they appear to be coiled around the pot and seem pot bound, gently pull the roots apart to loosen them. Plant at the same level as they were grown in the original pot. If the soil ball seems a bit large for the spot you want to plant, gently squeeze the soil ball to help it fit.
How many plants you choose to use depends on how quick you want a "full" looking container. If you want a pot that looks finished right after you get done planting, you can space the plants very tight. If budget is an issue or you can wait a while, space plants out with room between them. In a few short weeks, the pot will have grown together with little if any empty spaces left.
After planting, water thoroughly. This allows the potting media to get in and around the root ball of the plants and eliminate air pockets. Make sure water drains out of the hole in the bottom of the pot. You may need to water several times to insure the media is thoroughly moist.