Which kind of dog is the most well-behaved

When you say "messy or dirty," you have to realize that dogs do not clean up after themselves in any way. They do not groom themselves like cats, they do not pick up their toys around the house, and they certainly do not pick up their poop in the backyard! You will have to wipe their paws off when they come in the house on a rainy day, you will have to brush them at least once a week, and vacuum your house frequently. Even with hypo-allergenic dogs, you will not avoid mess and dirt.

Since you know so little about dogs and training, I would say don't even THINK about getting a dog unless you are going to enroll it in obedience class. These types of classes are very important for new dogs owners and their dogs. When a puppy is young, it is very important that it is well-socialized with other dogs and other people. Obedience class (in my opinion) is the best way to get both of these things accomplished. The dog will get used to obeying commands around other people and dogs. More importantly, since it is your first time, YOU will learn exactly how to train your dog. This is they key to a happy dog and a happy owner.

If you don't want a dog that is "messy or dirty" I would recommend a dog that does not shed. All dogs require regular bathing (no more than every 2 weeks as it will fry out their skin) and grooming. I would say to get a smaller dog that would require less maintenence, but many small breeds can have a tendency to be high-strung and hard to train in comparison to many larger breeds.