jorgelito, she is doing her job (instinct) by barking at strangers. She is alerting her alpha dog (you) that a possible danger is present. The wagging tail tells you that she is pleased with herself for being your protector, and that she doesn't intend to get aggressive.
You don't want to stop the barking all together, but you do need to teach her to stop at your say so. What has worked for us in the past goes something like this:
Dog barks, and you see who it is.
If it is a good guy (someone you know), give her lavish praise and say it is OK. Have your friend call her by name and hold out the back of his hand for her to smell. She should stop barking at this point and if she does, tell her what a good girl she is. Maybe offer a small treat for reinforcement. If she continues to bark after taking a good sniff. Gently, tell her No, bad girl. Let your friend in and put her on ignore. Let her bark her fool head off, but do not recognize her existence until she stops.
If it's a bad guy or someone you're not sure of, do not give your dog the OK to stop the behavior. Here's an example: Some door to door bible thumpers showed up at my door and the newfs barked away. I could have given the OK to stop, but instead allowed the dogs to intimidate with the barking. The thumpers decided it was best to leave. THEN, came the praise for my good dogs.
