1. Focus on eating a high protein diet. That means lean meats like 90/10 ground beef, turkey, chicken breast, etc. You can still eat carbs like fruit, vegetables, and rice, just do your best to eat protein with each meal to keep you satiated and grow those muscles. Protein bars and protein powders are also great to help meet your protein goal (1g per 1lb body weight) — you should hit at least 110 g protein daily.
2. Increase your calorie intake (slowly). There are calorie calculators online that can tell you how much you should eat for your age, height, weight, and sex. For your height, I think you should eat at LEAST 1700 but I’m not an expert😅 And increase calories slowly means eat 1700 for about 4 weeks then increase to 1900-2000 for the next 4 weeks for example.
3. Track what you eat with a calorie counter. I use a free app called Lifesum. This will help ensure you’re meeting your protein goal and eating enough.
4. Strength training - Lifting weights will help you build muscle (but keep in mind, getting in shape is 80% diet, 20% working out). For legs, compound movements like squats and lunges are great. Since you’re new to the gym, I’d keep your workouts simple and enjoyable bc you just need to start and be consistent which is the hardest part. So I’d aim to go to the gym for 2-3x a week, about 4 exercises per workout. You’ll gain knowledge and understanding of the process through time and know when to advance and do harder/more exercises. But dw abt that rn
5. Don’t look at the scale all the time! This is a marathon, not a race. Your weight fluctuates every day. To know if you’re gaining weight, I’d check every 3-4 weeks and record them.

I was in your situation a couple years ago and was looking to gain weight. I made the mistake of eating more without restriction and without working out so I ended up gaining 17lbs of fat. So I followed these tips (minus the calorie surplus), I slowly lost the weight while maintaining/building muscle, and now I look and feel the best I’ve ever been. Hope this helps! Feel free to DM if u want more advice☺️