1. Assess Your Sun
Here's the thing. You can grow gorgeous container gardens even if you have very little sun. You can grow spectacular containers if you are drenched in sun all day long (or anything in between for that matter). However, for your container gardens to thrive, much less be spectacular, you need to accurately asses how much sun your pot or garden will get. And here's a warning. If you just guess, or think you know how much sun exposure an area gets, chances are astronomical that you will be wrong--by a lot. No matter how good a gardener you are, the tendency I have seen again and again (ok, I'm guilty too) is to grossly overestimate how much sun an area gets.
The first thing you should do is figure out, either by timing with a watch, or using a sun calculator, how much direct sun your containers will get. You need to do this close to the time of year that you are going to plant, because in the depths of winter, the sun is in a different place than it is in the summer. The amount of sun your pots get will determine what you can plant in them. You can't know what will grow, unless you know how much sun the plants will have.
Making sure a plant has the amount of sun it requires to thrive is critical for any container garden
2. Choose a Container
Truly anything at all can be turned into a container. Anything from the size of a thimble to a parking lot can be used to hold soil and plants. However, here's the thing, for most plants, the larger (within reason) your container is, the more soil it will hold. The more soil there is, the more easily nutrients and water are retained and delivered to your plants and the less frequently you will have to water. Small pots dry out really fast and though some plants don't mind getting dried out, most do and are stressed by drying out. Stressed plants are more liable to be susceptible to pests and diseases. I use the biggest possible pots because I resent being a slave to watering and I want the most latitude for my mistakes.
When choosing a container, make sure it has enough drainage or that you can add drainage holes. I like to have at least a one inch hole, in a large(ish) container. If you don't have enough drainage, depending on what your pot or container is made of, you can usually drill, punch or pound extra holes.
Self-watering pots are great because they water plants, usually using a reservoir system, which also gives great latitude.
3. Buy Potting Soil
It can be confusing because sometimes potting soil is called potting soil and sometimes it is called potting medium, potting mix or container soil or mix. Just make sure whatever you buy is for containers. Do not buy topsoil or garden soil and don't try to dump some soil from your garden into your pot--you will be disappointed.
Just like anything there are good potting soils and not such good potting soils. However, most will work and for beginners, don't stress too much about it. Over time, you will find out what works for you and your plants. All the major brands that sell potting soil, will work. I prefer an organic potting soil and buy the kind that doesn't have fertilizer already in it. Either type of potting soil is fine--with or without fertilizer--but you need to know which you are buying.
Even if your potting soil does have fertilizer already in it, chances are as the season goes along, you will have to feed your plants anyhow.
4. Choose Plants
Ok, once you've determined how much sun you have, chosen your pot and gotten your potting soil, now the fun begins--choosing your plants. The first thing you want to do is to look for plants that thrive in the same amount of sun that your pot will get. Most nurseries have high sun requirement plants together and shade plants usually have their own spot too. However, there are tons of plants that are part sun, or part shade. So the good news is that whatever your sun requirements, there will be plants that will be satisfied, now you just have to find them and decide. Also, if you are doing a mixed container, you want to make sure that all of the plants you buy haven not only the same light requirement, but the same water requirement as well.
There are any number of container design philosophies, but the idea of a using a "thriller, filler and spiller," approach is great for beginners. Also, don't be afraid of putting only one fabulous plant, or several plants of one variety in your pot. Some of my favorite containers only have a single striking plant in them.
More on Choosing Plants
5. Plant Your Container
Oddly, this is the easy part of the whole process and probably takes the least amount of time. Once you have your plants, pot soil and fertilizer collected, cover the bottom drainage hole with plastic screening, paper towel or a coffee filter, so your soil will stay in and water can get out. Fill your container with potting soil to within an inch or two from the top. Now mix in fertilizer, carefully following directions for quantity (this is particularly important if you are using conventional fertilizers, which can burn the roots of your plants if you over use). I use an organic all-purpose, granular fertilizer. Make sure to mix it in well--throughout the pot.
Carefully take your plants out of their nursery pots. To do this without harming the plants, don't grab the plant and pull. Turn the pot upside down and push the plant out through the holes. If it's stuck run a knife around the pot, between the soil and the plastic. If you find that your plant is root bound, make sure to separate roots,. Arrange the plants, keeping in mind which direction your pot will be facing.
Dig a hole for each plant, deep enough so that the top of the soil of the plant in it's nursery pot, will be an inch or two from the top of the pot. You do not want to cover the crown (where the stem meets the roots) of your plant with soil, and you want enough room so that when you water, it won't splash out of the pot.
Fill in around your plants with potting soil, again, being careful not to cover the crown. You want to make sure there is soil surrounding your plants roots and that there aren't air pockets.
Water gently and generously, until the water flows out the bottom of your pot. After the first watering, you may need to add more potting soil, if holes or dents appear.
6. Keep Your Plants Alive
Ok, now this is the hard part--keeping the thing alive. Watering is key. And not just watering, watering the right amount. As a rule, your soil should be kept damp, not wet. To determine this stick your finger down to the second knuckle into the soil. You do this because the surface of the soil can seem or look dry, but if you stick your finger in, the soil under the surface may be wet. If your soil feels moist, you probably should wait to water.
Watering is particularly tricky because your pot will dry out faster on sunny days, and wind can suck moisture out of a pot. On cloudy or damp days your pot might not dry out. That said, it's easy to be fooled by a gentle rain. It can fool you into thinking you don't have to water, when your plant may actually be quite dry.
Depending where you live and how hot it gets, you may have to water a couple of times a day in the heart of summer, especially if your pot is small.