I'll break this up into a series of posts, I think, for simplicity's sake. But first, some garden basics. Please, if I get something wrong and you notice, correct me! I'd hate to spread misinformation, and I really do still consider myself an apprentice when it comes to gardening -- not even journeyman yet...
Perennial versus AnnualAnnuals only last a year (or really, a growing season), whereas perennials come back, year after year. Perennials tend to cost a bit more initially, but of course, they're much cheaper over the long-term. But some of the prettiest plants (and most brightly-colored and quick-spreading) are annuals. Most gardeners mix the two -- long-lasting perennials to grow slowly over time, cheerful annuals for exuberant color. It's really a question of your personal style as to what you pick. Do keep in mind that it's probably not worth trying to keep plants in window boxes or other small containers alive over the winter outdoors -- it's just too hard, because there isn't enough soil to insulate the plant from freeze-thaw cycles, and it's hard to make yourself water them sufficiently when it's freezing outside. So those are great places to fill with colorful annuals.
What Zone Are You?The first thing you need to know for buying outdoor plants, whether you're going to your local Home Depot or a fancy nursery, is what zone you're in. Chicago is in zone 5, basically. What that determines is what kind of plants can tolerate your weather conditions throughout the year. If I buy zone 7 plants, they won't make it over the winter outdoors. Higher zone numbers mean farther south (warmer winters).
Now, if you're just going to put some plants in pots and then bring them inside in the winter, that expands your options -- you can indulge yourself with lush tropicals that would never survive otherwise. (At the far right of this picture from my 2006 garden, there's a bougainvillea in a green pot -- note that it has a few pink flowering branches.) But it's a lot of effort hauling big tropicals indoors, and you need to have a suitable place to overwinter them. Some plants require darkness, like a root cellar, for healthy dormancy. Others are perfectly happy in a sunny window, though they probably won't flower (or not as much as they would outdoors). I have a jasmine that I bring in and out, and another jasmine that I just leave indoors all year. The bougainvillea, I love, but after we repotted it into a huge gold ceramic pot, it's now too big to easily pull in and out (lots of stairs), so we've given up on bringing it outdoors. It lives indoors for now, and we just try to keep it alive for the next few years until we live someplace where we can easily wheel it indoors for the winter. Then it should throw out tons of new vines and leaves, and bloom like crazy over the summer. We hope.
One thing to remember with containers -- you need to subtract a zone of hardiness for container plants, because the plants have a harder time surviving in them. So although Chicago is zone 5, I can only reliably trust that zone 4 plants will survive over the winter in my containers (and they may not all be able to tolerate the heat of our summers). That doesn't mean that I don't buy zone 5 plants -- it just means that I cross my fingers and hope the perennials make it. Ever year, a few of them do. So far, I've had a large hydrangea, some silver artemesia (aka wormwood) and some cheerful daisies come back after a Chicago winter with no watering (other than whatever fell from the sky).
Container Plants and WinterThere are a few things you can do to help container plants survive over the winter. Once a month, dump a bag of ice over the soil in the pot. This is easier than watering, and the ice will slowly melt during the day and water the plant. When you plant the plants, use a layer of insulating material, both at the bottom of the pot, and around the sides -- packing peanuts actually work great for this. Always mulch your pots in general, but in the fall, additionally, heap up some extra mulch around the base of the plant. And the bigger the pot you plant it in initially, the better its chances, because all that extra soil will help insulate it. I think there are other tricks (I want to say something about plastic bags?), but those are the ones I remember.
What Is Mulch?Mulch is any material (wood chips, leaves, gravel) that you spread over the soil. It serves a multitude of purposes -- it makes the planting look neat, it insulates the soil, it helps retain soil in dry, windy areas, and most importantly from my point of view, it helps retain water, so you don't have to water as often. Every time I plant a pot now, I leave room for an inch or two of mulch at the top. I like to buy big bags of cedar chips and use those -- they're light, even when they're huge bags, so I can carry them, and they're pretty. One bag should last you for several pots / window boxes / small planting beds. ALWAYS MULCH!
Those are some basic concepts. Any questions?
Next: Full sun / part sun / shade.