It’s a simple process of calories in versus calories out. If your calories in exceed those going out, you will gain weight. You don’t want to gain fat though, so you need to incorporate strength training into your routine to gain lean muscle. Muscle gain is particularly difficult for girls (ignore the myths that say girls bulk up with weights), so you will have to follow up each intense workout with a high-calorie meal, rich in proteins and healthy carbs. Make sure you are following the daily routine of having a high-energy breakfast, a substantial lunch and a slightly smaller dinner, all including healthy fats, proteins and complex carbs. Snack regularly in between, incorporating high calorie foods like oats, raw nuts, avo and biltong. Find a healthy meal replacement, and supplement your diet with this, using it as a high-calorie snack rather than replacing a full-meal.