hey there,well generally uhave to eat 3-6 times with nutritious diet.its should be rich in proteins,carbohydrates,antioxidants,minerals,vitamins.for example in breakfast ucan have half boiled or soft boiled eggs(1-2) with whole wheat/soy mix stuff,peanutbutter,any organic soy-based milk product.in lunch ucan have lean meat(salmon/fish produts) with legumes and nuts.ucan have fruit with that.evening ucan have any mixture of fruit juices.at night ucan have soy based meal with soups of different combos(veg-non veg with soy based bread prodcuts).make sure u have heavy breakfast if ur intake is 3 times and light dinner or supper.
Calories and proteins are what you are looking for. Some people think ''hey i will eat junk food and i will turn fat into muscles'' actually this is a very bad idea.
What you need is a good diet with healthy fats such as omega-3, essential fatty acids sources like tuna but personally i would prefer smaller fishes like sardines/pilchards because they contain less mercury, salmon ( mercury again don't eat too much ), flax, and walnuts. Good sources of protein include lean meats, fish, poultry, nuts, seeds and legumes. Healthy carbohydrate sources include fruits, vegetables and whole grains.
You might also consider buying a protein powder and drink shakes between your meals and after exercising. Personally i use QuickMass from Allmax Nutrition. I was was 118 lbs when i started it 1 month ago and now I'm 129.
Also if you are already used to exercise and have already built a correct endurance you can go for extra weight and less repetition. Something like 3X6 or 7 and wait 30 seconds between each repetition. It's not a good idea to start with heavy weight right on. So when you start you should be doing 2X12 or 15 with light weight then gradually increase weight then go for 3X12 and when you feel comfortable you can go for mass building which is 2X6 or 7 with heavy weights. To know what is the appropriate weight you have to be barely able to lift the 6th or 7th repetition. If you have no problem doing all the 7 repetitions then it's time to add weight.