Diegestive system improve. My liver?

Hi, according to my height I am drastically underweight. Since childhood my diegestive system is not good. The moment I have some spicy food or outside food, 6 out of 10 times I have a stomach upset. I work in the BPO KPO industry and into continues Night shift and have been doing the same for almost 3 years now. Basically from Calcutta, West Bengal relocated to Bangalore, Karnataka for job purpose. Kindly advise how do I improve my diegestive system and then enhance my weight to a minimum level. What should I follow? What excersise should I do? What food I should have? (I love biscuits. Since childhood I have had so many, so many biscuits) Protein, Supliment is it advisable? Any problem with my liver? Apart from this I don't have any issues. But this is the issue I have since childhood. Since my class 10 my weight is constant. It has neither increased nor decreased. Thank you for your suggestions in advance.