What size do you need? Consider how many people you'll be sharing your tent with on a regular basis -- plus how much other stuff they'll bring, including gear and pets. Will you have your car available for storing extra gear, or will you have to store everything in the tent? Most tents are rated for use in very close quarters; often, campers will buy a tent that's rated for one or two more occupants than you typically plan to share it with, so you can have a little room to move around and stow extra gear.
What will the weather be like? If you're camping in harsh conditions, a low, rounded design sheds wind, rain and even snow, although that limits the usable space around the edges of your tent. If conditions are milder, you can get away with a tall, steep-walled tent that maximizes living space.
Are you comfortable with a non-freestanding tent? Tents come in two basic configurations: Freestanding tents, which have some integral structure of their own, and non-freestanding tents, which will fall down if they're not staked out. Freestanding tents are easiest for a beginner to manage, but almost anyone can learn to manage a non-freestanding tent with a little practice. No matter what sort of tent you end up with, always practice putting it up before you hit the trail or campsite: You'll always be faster the second time through, and the trial run ensures that you have all the necessary pieces.
Will you be carrying the tent? If you must carry the tent a long way to your pitch site, light weight and small size suddenly become your highest priorities. A quick, easy pitch is important here too, since the tent will be your only shelter against bad weather.
Do you understand tent weights? Often, tents will have several weights listed: The trail weight refers to only the tent poles, rainfly and tent body. Packed weight includes all of that, plus stakes, guy lines, stuff sacks, instructions and packaging. Realistically, backpackers will end up with an actual weight that's somewhere between the two figures, since you wouldn't take the instructions and packaging with you on the trail.