Dogs bark. It is a natural behaviour for them but it can get out of hand and then it becomes a nuisance.
Attention Seeking
It may be that your dog has learnt that barking at you, gets your attention. If this is the case, you need to start ignoring the barking and rewarding the quiet. When your dog barks, pay no attention: break eye contact, turn your back or leave the area. If your dog follows, repeat. The moment the barking ceases; give eye contact, face your dog and praise it 'good!' along with a fuss and / or a treat. Then start introducing the word 'quiet' as the barking stops and follow it with 'good!' a fuss and / or a treat. The dog should soon learn what quiet means and will react accordingly. Commands shouldn't be yelled, it's all about positive association with you and this new 'language' remember?
Aggressive Barking
If your dog barks at someone or something while on the lead, immediately begin to walk in the opposite direction. Spin around and off you go. When the barking stops immediately walk in your original direction. You may feel a bit silly at first but you are showing your dog that you are in control of the situation.
Training To Stop Barking
You can also work with unwanted barking by introducing the 'watch' command. It gets your dog's attention away from the stimulation and back on to you. Start training the command in a relaxed environment. Holding a treat in thumb and forefinger near your eyes in a salute posture, say your dog's name followed by 'watch'. The minute you get the dog's eye contact, say 'good' & treat. Continue this a few times until it gets your dog's attention everytime. Slowly increase the period between the command 'watch' and giving the treat. Then you can start using this command in the barking situation.
'Watch' is a great pre-cursor command for all dog training; teach your dog it just for fun!
For barking when you leave your dog, see separation anxiety in the destructive behaviour article.
