A low carb diet but high in protein and veggies is probably a good start. Hard to get used to since we all have high carb diets usually. Once started though it is actually easy to maintain. In fact, you end up skipping almost the entire grocery store because all you go to is the produce for veggies (not fruit, too much sugar) and to the meat/fish counter. Also make sure you watch what you drink. Lots of drinks are loaded with sugar which affects your metabolism and also adds useless calories.

Starting some sort of exercise, even just a short walk will also boost your metabolism. I'm trying to do that myself and also using weights to tone up, but it is hard because I'm depressed. So go easy on yourself. It is a difficult process for anyone to do.