After being a thin teenager at one point I have some advice for you. Don’t worry about carb cycling at all. In terms of nutrition you first need to get your protein intake up to at least 1g per pound of body weight. Try to use good sources (ground beef, eggs, chicken breasts, whey, cottage cheese etc) Next you are going to want quality carbs (rice, bananas, fruit, pasta). Also don’t forget good fats (peanut butter, nuts, avocado). Get at least 3 to 4 decent sized meals every single day. For calorie goals I can’t be precise but somewhere around 2500 would be a starting point. When it comes to fluids try to drink a gallon of water per day.
In terms of training find a beginner plan you will be consistent with. If it’s a 4 day plan you better be able to make it all 4 days. Full body or upper/lower would be good. Watch videos on proper form and work at increasing the weight you use. Sleep will be critical. Get 8hrs per night.
I know you think you would like to lose some fat. If you stick to this plan for the next few months you will put on a significant amount of muscle quickly. All of a sudden that fat won’t be so obvious once there is added muscle. To finish this small book I would add make sure to live your life. Your 15, chase girls, go for late night fast food with your friends, drink stolen beer on weekends etc. Best of luck
(I just realized the name on the post is Ashley, if this happens to be for a female I would change a few things. Lower the calls to 2000 and chase boys haha)