[muffinman26]

Make sure you're eating enough.

When I find myself not having the energy to go to the gym, it's usually because I'm not eating enough. A lot of people get into fitness to lose weight and you scientifically need a caloric deficit to lose weight, so a lot of reading about diet/exercise suggest low calorie diets, but for fitness exercise is more important and that requires calories to sustain. Low calorie diets turn me into a slug who can barely focus at work, let alone exercise.

Make sure you increase your calorie intake when you start exercising regularly. Don't worry too much about where you're getting the calories from at first. Drastic diet changes take time to get used to, and you need the energy to think clearly and experiment to find what works best for you. If you want to eat healthier you can make incremental changes towards less processed foods/lower sugar and work towards a balance of fat, carbs, and proteins. (A good rule of thumb I've seen is 1 gram of protein per day per pound of target weight, with 100 grams/day as a good starting point.)