Diet is 80% of the battle. Get a good diet in place to assist with cutting down on fat and increase on your protein with a medium carb intake. 40/40/20 split being protein, carbs, fats. Make sure you have a good water intake during your day. When training don't go to heavy but rather keep it at a medium weight that you can do 15 reps for 3 sets with. Don't go to slow but make sure your technique is good and you keep the reps at a nice pace. You want to make sure you are getting a nice pump with a good heart rate to burn fat but still activate the muscle growth. I would suggest a warm up on the bike or treadmill at 60-80% effort for 5 minutes before and after the weight training to assist with getting your heart rate up and firing up your metabolism. Please note that this is purely from my experience and I am not Aware of your measurements or medical conditions.
Genetics has a lot to do with the length of your muscles! and many other things. You wont build muscle without training!