Pasta, wheat bread, brown rice, oatmeal, bagels, avocados, brocolli, vegetables in general, fruits, almonds, milk, plain yogurt,etc. Incorporate these things into your diet and you will definitely notice results.
There are a couple of factors to gaining weight. Eat big but eat clean, lots of whole grains, lots of leafy greens, lots of lean protein, simple carbs in moderation. At least 5-7 meals per day. Snack on things like almonds and yogurt, things with good calories and high nutritious value. If your numbers of calories in is greater than your calories out, you should be gaining weight. If you're eating is clean, you should be gaining lean muscle. Lift BIG. Lots of guys think they lift heavy and really haven't scratched the surface of what their body is capable of. Big weight = big muscles. And get AMPLE rest. 7-8 hours of UNINTERRUPTED sleep per night, and on your days off at-least have one day where you barely move. Your body needs to heal to grow.