Dogs bite for quite a few reasons. They feel stressed, scared, startled, threatened, etc. They may be trying to protect a favorite toy, food, or anything else valuable to them.

If your dog is already biting people, hire a personal trainer. Seriously. You have a lot of work to do if he has already started biting, and no amount of Googling or community advice is going to give you the amount of help you need. He can unlearn the behavior, but you need professional help.

That said...

Make sure your dog understands you are the alpha. Always make sure you are entering the house first; you can even have him make him sit and wait before entering and leaving the house. If he barks at the window or door, calmly walk over, look at what is outside, tell him "good, thank you", and walk away. This way, he's alerting you to something, and you're showing you're not concerned, and you've taken care of it. Teach him the "Leave it!" command. Start with toys and food. I have my dogs "sit" and "leave it" for every single meal until I release them to eat. I also make them sit before giving them any treats or toys. Don't let him jump on you or furniture uninvited.

Use walks as training opportunities. All of the walks. Carry training treats on your walks. Keep him close to you. Watch him carefully. Every time he looks at you say "good" and give him a treat. This teaches him to follow your lead. If you see someone approaching, start giving him treats to get him to associate people approaching with good things. Check your emotions. He can sense what your reaction is, so if you start getting nervous or anxious because you're anticipating his reaction, he'll pick up on it and interpret it as an approaching threat. Move to the side and stand in front of him with your back to the approaching person. Have him sit and give him a treat. Continue giving him treats while he sits, and keep standing between you and the approaching person. Continue giving him treats until the person passes.

If you are using a retractable leash, stop. They encourage pulling and give control to your dog instead of you. They are horrible inventions.

Good luck!