|Read the label.|
The label will also tell you the dimensions of the plant when it reaches full size, which can guide you in selecting the proper size pot. It will also tell you the plant's hardiness zone. If it's a perennial and you have hopes of it surviving the winter, be sure it's hardy to your zone. I usually subtract two zones because plants in pots are more exposed to the elements. Here in Virginia, it's zone 7 so if I have any hopes for a plant to return the following year, I look for those that have a zone of 5.
|There's good information here.|
Finally, if it's a flowering plant you're looking at, the tag will tell you its bloom season. With annuals, you can rely on blooms the entire season. But with perennials, you'll want to read the label closely. Most (not all) perennials will only bloom for a couple months--it might be spring, early summer, mid summer, late summer, or fall. You can plan accordingly and buy different types of flowering perennials so that you can have staggered blooms. I often mix flowering perennials with annuals so that I'll always have something in bloom. But keep in mind, flowers aren't everything. I've learned to appreciate interesting foliage, which can often be beautiful all on its own.
So read that label and give your plant the best possible chance to thrive.
Bookshelf: Container Gardening Books