Always remember that no sugar doesn't necessarily mean something isn't harmful. Diet sodas are one of the worst things you can put in your body, and arguably even worse than their sugar-laden originals. Artificial sweeteners, coloring, and other questionable ingredients all feature in your favorite "diet" drinks. The caramel flavoring in diet cola drinks has also been found to increase the rate of tumors in rats and has been flagged as a potential carcinogen for humans.
Though they're marketed as zero calorie, "healthy" alternatives to regular sodas, many of them have been found to increase glucose levels and add more stress to the liver, which leads to more fat production in the body. If you're looking for an alternative to soda, try getting into herbal tea instead. It might not sound as exciting, but you'll get used to it!