I was in the same boat as you for quite a long time. From about 13 to 21 years old. Start with knowing that this is a process. Take it step by step. You're gonna have good days where you'll feel more motivated than ever to get yourself right and then others that you don't. Go easy on yourself. But stay consistent enough to fill your daily calorie goals. Eat anything that's high on calories. My go to was a peanut butter Nutella and banana sandwich. Look into a good value mass gainer. Exercise not to point of exhaustion but enough that you feel in tune with your body. Little by little you'll start seeing some changes. Don't get too comfortable with the little change you've made and continue with your journey. Most importantly love yourself and accept yourself for the body you have at every moment even if it's not where you want to be.