Not every dog is right for every person, and knowing which type of dog works well for you will help ensure that you have a lifelong and happy companion.
Do you like active dogs? Are you planning on using your dog for a 'purpose' other than just companionship? If so, a border collie, labrador, or golden retriever may be your best bet. These dogs love to work, and love to snuggle with their families as well.
Do you have small children? Do you need a calmer dog? A dog that is great with smaller kids is a basset hound. Another great dog is a golden retriever again, as they are patient and great with kids.
If you are wanting a smaller dog, try a West Highland terrior or a maltese. These dogs are quiet and easy to please.
If you want a protective dog, go for a German Shepherd or a Great Pyrenees. These dogs are loyal and friendly, but don't mess with them.
If you don't know much about dogs, a golden retriever may be your best bet. They are great for many 'uses' in the family, and are docile and fun dogs that any family can enjoy.
Just make sure whatever dog you choose, you are ready for at least 8 years of commitment, as dogs are not clothes you can just give away when you outgrow them or they aren't fun anymore. Do your homework so you choose the best dog for your needs.