I do think a training class is a must for most people since, as I said, the timing of reinforcement is the biggest cause of trouble I see.
This is the key. I've been through training classes with a half dozen dogs, and each trainer stressed that training people is more important to them than training dogs. As Mac said, everyone in the household has to be consistent, otherwise the dogs gets confused. The biggest hurdle to training dogs is learning to think like a dog. They aren't small people, they are domesticated wolves. Dogs evolved from the wolves that were people friendly, and they want to be told what to do. Left to their own they will make their own rules so it is important that you and everyone in the family set the rules and enforce them consistently. By enforcing I mean rewarding desired behavior with treats, love and enthusiasm.
Unfortunately, there are no real credentials, and anybody who wants to can print a business card or write a book and sell dog training advice. The obvious is example is Cesar Milan, who is rich and famous and a complete idiot in the view of almost everyone with any actual training or credentials in animal behavior. So when you look for resources, be wary of rigid rules, exaggerated claims, aggressive techniques, and things that just don't seem to make sense. Most mainstream training classes are fine (local humane society, Zoom Room and other chains, etc.), but there's still a lot of stupid out there.
have fun!
I recommend watching a few minutes of a Cesar Milan show so you'll know what not to do around dogs.
Edit: My wife got home and reminded me that dogs are excellent at picking up on patterns, hence the need for consistency.