First, she is hitting the second fear imprint stage. This will last a few months and is extremely important that you take good care of protecting her when she is uncomfortable.
The growling at strangers - this is in no way aggressive. So take that word out of your vocabulary. 99% of the time, this is not aggression, but fear - instead it's there way of showing their discomfort and she is at the age where vocalization comes in. It is best NOT to force her in any way to meet people. She should be out and about, but keep strangers at a distance until you see her "come out of hiding", like behind your legs. Allow people to approach, but tell them to ignore her. She may wonder out and take a sniff, and then run back again. Take lots of tastey treats, like hot dogs or cheese, and have the strangers drop a bit or two for her. She will learn that new people mean good things happen. DO NOT allow people to touch her, especially on her head until she is completely comfortable and this may take a few weeks/months til she is. NEVER push her to do anything until you see her WILLING to investigate.
Let her make the first move towards people. If you see her ears back, tail tucked or "squatty" stance, remove her from the situation. She is giving all the signs of being fearful. Don't hide her away in the house, just make sure you concentrate on her body language - not the people around you. After a little time, when you see her getting braver, then you can allow strangers to kneel down, side facing her - no talking, no reaching out, no looking at her....just kneeling down to her level. Put a few treats by their side. Allow her to approach at HER PACE. By this time, she will probably be a typical wiggling boxer butt and then slowly the stranger can interact with her.
Every experience she has from now until about 1 yr old will shape how she will react to situations in her adult life. If you take the time now to allow her to go at her own pace, she will be a much less reactive adult dog. If you rush her, then she may end up using all her tools in her toolbox that she accumulated during her uncomfortable first year. For instance, forcing a puppy to be handling too quickly when they are obviously showing signals of discomfort, can cause them to react with growls. Stopping them from growling, which is there way of saying "hey, I don't like this", can then develop into no signals and an instant bite. You WANT her to show signs of her discomfort, and growls are part of that. Barking is another form of their communication. So when she starts that, you need to figure out what the cause is.
If you keep your eyes on her body, behaviors and signals now, you will be able to assess her behaviors later. If you walk her without paying any attention to her, you risk losing all that reading of her signals and may miss them when they are so obvious later - again could be all the signals, you ignored them, and she reacts out because you weren't paying attention when she was telling you by all the means she knows how.
I would suggest some reading - Patricia McConnell has some great books on reading body signals and how to watch and read your dog. Ian Dunbar is also a great read, lots of info, and master of the positive training methods. Reading about clicker training can be a valuable source when training and overcoming social obstacles. There are several others. But please stay clear of any types of "dominance methods" training, like jerking leashes, grabbing the dog, hitting the dog, rolling the dog. You can gain a lot of trust between you and your pup with lots of praising, rewards and positive reinforcement. There is no reason to train a dog with using your hands forcefully on them it's all about your body language to them and using your logical thinking. Think through your reactions about bad behavior, BEFORE you react to them. Us humans are very poor in our reactions when things don't go the way we expect them too. Training dogs in a postive way, also teaches us patience and perserverance......a great value to have!
LISA ~ Lily LaRue - 4/07 boxer; Buster McDoogle - adptd 6/07 Vizsla/Besenji mix; Annabella Kanicki - 5/08-7/09 staffy; Mr. Smiley - 4/10 Boxer/pibble mix; Fozzy Bear - 5/10 Red Heeler/pibble mix