Perennials
If you want flowers that will come back every year, learn more here
Before buying perennials, you shold familiarize yourself with your USDA zone, especially if you're ordering from a catalog.
You will also want to pay attention to sun exposure in your yard. A common mistake for the new gardener is to think an area is "full sun" becuase it receives some sun during the day. However, if you have trees, or if you're planting by your house, you have to look at shade at different parts of the day. If you have trees, your yard is probably either part-sun or shade.
Big box hardware stores can offer good deals on plants, but you're probably not going to get much guidance from store employees, like you might in plumbing or hardware areas. If you want advice about your plants and want to make the best choices, you might be better off going to a nursery to buy your first plants, unless you have an experienced friend to help.
When planting a perennial, a good rule of thumb is to dig a hole twice the size of the root ball. Then, soak the plant thoroughly to help establish roots.
If your perennials are established in a good location, you may not have to do much besides water in a drought. If the soil quality is poor, or if the plant is getting scorched by sun, the plant will need extra water. If you find you have a "needy" perennial, you may benefit from digging it up and moving it.
If you find that you need to move a perennial, think carefully about what you believe the issue may be. You don't want to stress the plant by relocating it again. Make sure you dig up the whole root ball and soak the roots as if you're planting it for the first time. If you're uncertain about a better spot, you can probably use a pot, but make note that the pot will hold less water. More watering will be required. Unglazed terra cotta holds water a lot less efficiently than plastic, so unless it's a succuelent, I'd suggest avoiding terra cotta pots. Glazed terra cotta is better.