I want to know what pills or vitamins would help me to gain weight?

depends on the cause: Several things can make it hard to gain weight. The main ones are inadequate intake of calories, need for hydrochloric acid ;/or digestive enzymes, malabsorption, inflammation of the intestines (possibly from intestinal parasites), celiac disease ; other food allergies, vitamin/mineral/amino acid deficiencies ; hyperthyroidism. Pills that address the cause should help. See a good doc to evaluate.