The first consideration when purchasing or making a container is size. Soil is not just for support. It also is the main food and water source for the plant. Quality soil is a must and it can be amended with mulch. Check packages at your garden center for soil that is made specific to container plants.
Keep in mind that plants grow roots several feet deep but the top part of the root is the real powerhouse and the rest are for emergency water. In the container, the root systems will grow shallowly and do just fine. This is why container plants are just as easy to grow as any in a typical ground based vegetable plot.
That being said, size does matter. Some vegetables, even small patio tomatoes, need a 12 inch depth for the roots. For proper nutrition supply, though, put those in a pot that will hold a total of five gallons worth of soil. Peppers need about the same as patio or bush tomatoes. Squash and cucumbers will have differing needs since some bush types claim to save space but are not always small enough for containers. When purchasing these summer fruiting plants, make sure the catalog or distributor says they are okay for containers.
Lettuce and green on the other hand, can take to even a shallow window box. Best depth for these is eight inches and you can inter-plant with small bush type herbs like parsley and coriander. Herbs in particular thrive in containers with little more than water and sun to get a good harvest.
Watering should be consistent. Pots dry out surprisingly fast, and in the hottest of summer will need a quick second spray in evenings. Clay pots in particular, even glazed, can evaporate water from the sides so check these daily if possible or reserve pottery planters for drought tolerant flowers.
Nutrition for plants varies by type. The best way to start your container plants off is to use fresh soil made specifically for the plant type and organic mulch. Apply fertilizer throughout the growing season, again, based on the grower's instructions.
Gardening in containers is a very easy alternative to a ground based plot. Containers save space and you can move some indoors to extend the growing season. If you don't have much soil space, or if you want to decorate a deck with plants, container gardening is a great solution.