It's harder and easier than it sounds at the same time. If you want to gain weight, you're not going to gain it from nothing. You have to eat more!
But you have to eat more fruits/vegetables and less of that fatty stuff.
I found that when I was bulking, I was living in the dorms for my college career. Since they had a buffet style, all you can eat, rotating menu every week, I could really eat my fill of anything I wanted.
I really stocked up around the salad bar.
I ate lots of corn, spinach, peas, watermelon, pineapple, stuff like that.
I really used it to my advantage, and made sure to make the best of the meats that were offered; staying away from pizza, fried chicken, and things like that.
They would often have dark-meat chicken, and I know it's not the best for staying lean and gaining mass, but if you're really working out as hard as you THINK you're working out, it should be fine. I ate it a lot whenever they had it, and was happy with my results.