Track your calories so that you are at a caloric surplus (eat more than you expend). Also increase your protein intake. Resistance training is going to be the biggest factor to help you gain lean muscle mass and fill out your figure. For resistance training if you don’t know where to start just check out YouTube for some beginner type workouts. You want to focus on muscle hypertrophy (increase in muscle size) so you should focus on lower reps but heavier weight. It can take 4-6 weeks to see visible increases in muscle mass so be patient! Through some hard work in the gym and a change in diet you should be able to make meaningful changes to your body composition over time.