Are dietary supplements good for me?

Most aren't needed. An over-the-counter adult multivitamin (+/- minerals) is fine for a teenager to take once a day. If a teen doesn't eat or drink much in the way of dairy products, then an adult vitamin D and calcium tablet is also needed. Some people just take a TUMs Smoothies once a day for their calcium supplement. Vitamin D and multivitamins are even available in "gummy" chewables for adults now but have sugar.