There are three things which are very important for you to do:
The first is to keep your dog as slim as you can. Boxers tend to be chunky dogs, and people often think they look good like this. Obviously it is up to you, but ideally Boxers should have a very "tucked up" abdomen with a skinny waist. I would aim to keep your dog slim throughout the growth phase and beyond. The slimmer she is, the better her hips will be.
The second is to feed a carefully balanced diet. I would recommend you feed one of the good quality, complete diets, appropriate for the age of your dog, and never give any vitamin or mineral supplements and virtually never give any other foods. The reason for this is that the manufacturers of these diets work very hard to ensure that the vitamin and mineral content, and the balance between the different things, is at an optimum level. For instance, if you were to add extra calcium, you might think you are helping, but it could upset the balanced proportions of Calcium, vitamin D, phosphorus, and protein . People often think they are helping by adding something extra, but usually it just makes things worse, and you really will not do better than the skilled company nutritionists do. The only supplements I would recommend are those containing glucosamine, chondroitin and Green Lipped Mussel extracts. These will not upset the balance of the other ingredients and might have a beneficial effect on the joints.
The third thing to do is to keep exercise to a sensible, restrained level. Hip dyslpasia means that the ball and socket joint of the hip is a poor fit. This results in a "sloppy" joint which causes the ligaments around the joint to be strained and very slightly damaged regularly as your dog exercises. You might not notice much, other than slight weakness of the hind legs, but the cumulative effect of this minor, regular damage is arthritis. Essentially this is how arthritis usually develops. If your dog gets tired, she will be less able to protect her joints from the sudden movements which occur when she misses her footing or slips or trips at all. When she feels tired, she will be much more prone to this sort of damage. Therefore do let her have fun (this is important of course), do exercise her, but put the dampers on things if she is over-excited and do not make the exercise over strenuous or prolonged. Ever.
The good thing is that I have known lots of dogs with hip dysplasia do very well. If the simple rules above have been followed, I have known them to live long, full and happy lives and to keep good mobility into their old age. It is best not to feel disheartened by this, but to bear in mind these important things and she will probably have a very good life.