Can you afford a pet? Whether you get a cat, dog, or anything else, you'll have to be buying food constantly. You'll probably need to get a cage for a dog for at night or for when you're not home. You'll need a few toys for your animal. You'll have to get a food/water dish. If it's a cat you'll need a littler box and scooper and have to buy litter whenever you run out. If they're sick they will need to see a vet, which can be expensive.
Also, what type of pet exactly do you want? Dogs can be fun and playful and even a "watch dog" for you, but you will have to potty train them and take them outside in any kind of weather for them to go to the bathroom. They'll need lots of attention and have to be taken on walks. They can be sometimes loud when barking say if they look out a window and see a squirrel or something. Do you have any kids? Is this dog a puppy that can learn how to behave around a child? Is it an older dog that is already good with kids? Or you can choose a cat. They're much more independent and don't need quite as much attention. But you will have to keep up on their food and water and cleaning out their litter box. Or maybe you want something smaller that you don't have to do much with, like a hamster, a fish, something like that. But then again... you do have to be cleaning out the cage or fish tank and changing their food and water.
Do you have time for them? Can you make sure you will be able to take the dog out when it needs to keep track of food and water and keep up with changing and cleaning the litter? Will you have any time to play with them and give them attention?
If you can afford it and you have the right amount of time for them, then all you have left to do is choose which type of pet you want, get the necessary items for them (food, dish, litter box or cage, toys, etc) and then go and pick out your pet. Just be sure it's the right pet for you and everyone living with you.
If you do get a pet, just be sure to love them and take care of them, as there are many people out there who don't. Good luck with your new friend.