i want to change my life style and start eating right

So i am 21 years old. I have a high metabolism and anything that I eat I can easily not gain it. I eat crazy like I could eat a whole cheeseburger and fries and soda and not gain a pound i’ve been seeing a big difference in my body lately I have a little fat but I am skinny but I plan to eat healthy and have more energy because lately I don’t even have energy to go up the stairs any suggestions on how to change my lifestyle and meal plans and I plan to have a flat stomach and a big ass any tips?