Orthodontic patients are considered at risk for a number of reasons. The teeth are harder to clean when there is a metal device glued to them, and the teeth are in a state of inflammation, making the situation extra hard on them. This is why your teeth are at risk during orthodontry, and why you should take steps to minimize the risks.
Make sure you get all of your cavities filled and treated, and do not be too cheap to deal with pre-cavities either. These little demineralized zones are very likely to turn into cavities. The dentist can apply some fluoride and calcium mixture to the area and remineralize it. It is especially important that you take care of any and all root canal treatments that you may need to have done. The force of the device will almost certainly trigger a painful reaction, and getting dental work while in braces is a lot more complicated, costly, and takes longer than without them.
It is not just recommended, it is absolutely MANDATORY that you get a hygiene session before the braces go on. This is the only way to minimize or eliminate the possibility of plaque and other nasty stuff working its way into gums or damaging teeth during the orthodontic procedure. You need to have your teeth be at their best when the braces go on, as orthodontic devices tax the teeth, the gums, and the periodontium.
Make sure that your gums are clean and without any sort of bacterial infection whatsoever. Since your gums are going to be inflamed for the next half year to two years, you really should go into it with no bacteria, no infections, and no problems with your gums, or you may find yourself in a tight spot. Usually a course of antibiotics and a hygiene session takes care of the situation, but occasionally the course needs to be repeated, and even oral surgery may need to be employed in order to clear your gums of periodontitis. Best to take care of this before the orthodontic process begins to avoid stopping treatment to deal with a problem halfway.