I know what your saying. I use to be the same way. I weighed 125 for a long time. Went to gym lifting weights and gained muscle mass and got up to 150. Make sure you take a protein shake within 15 mins after your workout. I also take creatine with my post workout. I use No shotgun as my pre-workout. 4 to 5 days a week and mix it up. Working the same muscle groups every workout will give you no results. Legs are a must for lifting and the stronger they get the more mass you will gain. Also eat 5 to 6 meals a day. 3 main meals and 2 or 3 snacks depending on the workout schedule. Make sure to eat every 3 to 4 hours to keep your metabolism going. A snack can be a protein shake with peanut butter. You should take into consideration how much protein you have in your meals. Think of it as 1.5 grams of protein per pound and this will help you gain mass. You can also substitute a mass maker as a meal. Check out sbmuscle.com and I use gaspari nutrition for my protein and optimum nutrition for the creatine. Hope this helps you as everyone is different. I can give you my workout plan and also the muscle building diet if your interested. Also make sure to count your calories. Your diet will be 70% of your workout plan. You body will burn calories everyday so just find out what is and increase your calories.