It sounds like you definitely need a trainer to come in and train you how to train your dog.
You can read a ton of books and get online advice, but there is no substitute for having a trainer with you and your dog. Clicker training is an excellent method and is what I use for everything I train. It is a positive method and is very effective.
The way you explain her behavior towards guests sounds a lot like she is fearful of them. The tail wag does not mean she's happy, her body language is probably saying something different - something a trainer could tell with a visit. The barking at a stranger on the sidewalk until she can't see them is also a form of reactive behavior. Without seeing her you can't really tell if it's excitement, fearful or otherwise.
I would check the APDT website and/or the
CPDT website for trainers.