Just weightlifting is not going to make you gain weight beyond the "newbie gains" you might see (i.e. swollem muscles). Weightlifting is just the same as cycling, it is an exercise and you are burning calories. In order to gain any sort of lean muscle mass, you will have to consume more calories than you burn. But eating calorie dense, crap food is not going to help you. Just eat MORE of everything you are currently eating. Clean foods like whole grains, lean meats, tons of veggies, some healthy fats and you should have no problem gaining some weight as long as youre lifting correctly. You might want to try investing in "Weight Gainer" protein powder if you are really having difficulty gaining weight. Its calories packed, but usually without all the processed sugars and saturated fats in ice cream and things you are considering. The goal is to gain lean mass, not flab right? Fitday.com is great and usually pretty accurate for keeping track of your caloric intake. Good luck.