Put up sticky yellow cards, to trap the adults. Spray thoroughly and frequently with insecticidal soap, or organic dish soap solution, to kill the nymphs. Neem oil or extract will repel adults from new growth, and derail the development of the nymphs. If you can find out which species of whitefly is on your plants, you can order parasitoid wasps to reduce the population. Many chemical insecticides have been used on whiteflies, and they are still sometimes effective, but many species have developed resistance to multiple insecticides, and are only stopped with the organic methods, anymore.

I used to place pots of Tagetes (French Marigolds) next to each plant, and that was very effective. I hate the smell of them, so maybe the whitefly did too! ;-)

That works on some species of whiteflies, but a few actually like the smell! : /