When choosing your dog, it's important to keep in mind what kind of space you live in. For example, if you have a one room studio where your bed also serves as your dining room table and you do your dishes in the bathtub, a Great Dane would not be your best option. If you have a roommate that's allergic to them or live in a building that doesn't allow pets, then (surprise!) don't get one at all. Birds, reptiles, and rodents are often acceptable pet alternatives to both situations. If you're really desperate for some dog love, volunteer at your local animal shelter.
Also keep in mind your available time. Remember when you were little and you begged your parents for that dog and promised you would feed it and walk it all the time and then once it got to be older than a puppy and you got more homework and more friends your parents had to start taking care of it? Well, unless you still live with them, there's not going to be anyone else to take responsibility. And it will be your--and only your--room covered in dog poop. So, be realistic about how much time you can devote to training and entertaining your new fuzzy friend.