Eat healthy food's, exercise and take a lot of water.
I have to disagree with the advice to eat high calorie foods to gain weight and to eat cream and milk and not to drink water -- this isn't healthy. Weight gainer shakes aren't healthy. What you want is a healthy diet that accomplishes what you want to accomplish -- putting in unhealthy food doesn't do that, it just makes you fat in places but still the same overall shape. There is also no vitamin to gain weight, and doctors know virtually nothing about vitamins. Look, really, it's not that hard to figure out -- anyone who wants to gain weight in a healthy fashion can do it in two ways -- eat more healthy food and do resistance training that puts on muscle -- so that you gain weight but don't get fat or become unhealthy.
Just keep eating healthy foods, take a vitamin, ask your doctor what vitamin is good for you.
If you want to gain weight, then you have to add high calorie foods into your diet. Don't drink water before taking meals. This will fill your stomach and make it harder to get in enough calories. Eat more often. Drink milk. Try weight gainer shakes. Add cream to your coffee.
This question often pops up, so checking the archives on this forum will give you a lot of suggestions. My advice on this question of gaining weight is always, eat more of the healthy foods you're already eating even if that means adding a meal. Now, you're profile says you're female, so I'm suspecting you're not actually female. I also don't understand the working out part -- what do you mean you find it difficult? And remember, you're only 16, so don't go nuts on anything -- you're going to fill out and change over time and you're too young to go to any supplements for this. Most 16 year olds, unless they're serious athletes, don't work out, they play. Do you exercise at all?