If i gain muscle, will it build below my fat and make me look fatter or will it eat my fat?

Muscle is not fat: Muscle does not "eat" fat. Fat represents your body's energy stores, and if you are overweight it means you have too much energy stored up. The act of building muscle (ie. weight training) can help to utilize excess energy, but actually it is aerobic exercise (cardio) that requires more energy. If you work out regularly and diet, you'll hopefully build muscle AND lose the fat.

Hopefully: You will lose fat by attending to diet and exercise. That way the muscle you build will look more toned. It is possible if you end up building more muscle and losing less fat that you might look bigger. For specific advice re body composition and goals recommend you consult a qualified trainer, physical therapist or sports physician licensed in your area. All the best.

Muscle : It will build under the layer of fat if you do not follow a healthy balanced diet. If you follow a healthy balanced diet over time muscle will burn more calories and fat but diet is 85-90% of how your body looks. Read this link: https://www.bodybuilding.com/fun/fat_loss_muscle_gain_trick.htm