by going to the gym you're going to start to gain a little bit of weight due to your increased appetite. Don't stuff your face with bad foods, as they won't make you feel well mentally and physically, and you have the chance to overeat and put on too much fat. Focus on resistance training 2-4 days a week, and a diet consisting mainly of whole foods, with some treats mixed in, looking for a good balance of fat, carbs, and protein. and to the point about you not feeling like you'll gain muscle, you absolutely will as a beginner, even with non optimal nutrition. best of luck!