You've been given some good advice. The best so far I have read for you is that you do not get into this field unless you are at least studying HVAC. In fact no one in my state will be given permission, bus lic, ect. to do any work duct cleaning or other wise unless you possess the requires hvac contractor lic. Which requires one to know the trade along with other bis. information. And this is for good reason. Because in order for you to do the job correctly its going to involve more than just sending a brush down the supply floor vent. That said You need to check out NADCA. National Air Duct Cleaners Association. They are the group who writes the standards for this field. I would take them seriously, as there are many contractors out there who do not understand the need nor the know how, and many don't want to spend the time nor the money in order to do what is necessary. If you get in touch with these guys and respect what they have to teach you and keep yourself from making the same mistake as do so many others you'll do fine.