Well eating is very important if your looking to put on weight. I myself was never skinny, just lean, but I wanted to put on some mass a few years back. All you have to do is aim to try and eat 1.25 grams of protein for every pound you weigh, so since you weigh 140, gun for around 175 grams of protein a day, your best bet is to invest in a good protein supplement that has at least 35 grams of protein in it, then mix with milk, which usually has 8 grams in it a cup, mix the shake with 16 oz and you have about 50 grams of protein right there, eat at least a can of tuna a day, which has about 35 grams of protein, then make sure you eat meat with your lunch and dinner. Make sure you look on the labels of everything you eat, it will help you in your goal to get protein in your body. If you would like a workout plan or more diet tips, just PM me, I myself was able to get from 175 pounds to 200 pounds in 5 months, and trust me, I was a hard gainer. My arms went from 14 and a half inches to 16 inches, my chest went from average to pretty big, and my already pretty big back got wide as ****. While I did lose my abs, my body really filled out in the end, and I ended up having a pretty nice body that looked great in a beater.