Getting energy from food isn't as complicated as it sounds. For a simple and easy to understand overview of how this process happens we have linked the following button to an interactive chart of how it all comes together. Spend 5 minutes studying it and impress your friends with your in-depth knowledge of the metabolic pathways!
MyFitnessPal is a website that you can go to for free and keep track of your calories in and out. While I wouldn't necessarily deem it important to count calories because I've been losing weight quite happily without doing so, I found it handy for helping someone who has stalled on their weight loss plan. It helps by making you a bit more aware of what the effect of a snack might be and alternatively how you can just go for a walk or a bike ride and lose that net gain from your snack. I wouldn't go to much by the recommended fat and carbohydrate targets, since if you are like me you want to get rid of carbs from your diet and get your calories from fat.