I've been lifting weights for the past few months. what can i do to increase my body mass? what is a good way to do this?

Calories, heavy wts: The best way to gain body mass is to eat, eat, eat. You need more calories then you burn off. For lean mass, you won't to watch the kinds of foods that you eat. You don't just want to eat pizza, donuts, soft drinks. You should shoot for lots of healthy proteins, healthy fats, and nonprocessed carbohydrates. As for the weights, shoot for heavy weights and low repetitions.