Just buy a lot of milk and eggs and eat at home n between your 3 main meals. If you have some extra cash, feel free to get urself a tub of Whey protein to eat during your postworkout or preworkout, or even supper.

Remember u need minimum 5 meals per day, spread evenly through out your day, and it must be good food not junk like roti canai or 3 bags of popcorn.

With that said, if you, an ectomorph, feel like doing cardio for the sake of general cardiovascular health, just go ahead and do it. Its fine to do cardio, as long as you don't overdo it. What is considered as overdoing depends on what you think is considered as overdoing... You'll know.

Bodybuilding is like a product life cycle graph. In the beginning your strength and mass climb rapidly, and then after a while growth begins to slow down and changes become less noticeable. So don't be too sad about it, just keep eating as much as you can, every day, and be consistent with your workout