One of simplest thing which you need to start in order to gain weight is to increase your calorie intake and start with a good work out plan to build muscle. So you need to start taking more calorie rich food. It will help you in adding muscle and also body mass.
For this purpose you can buy the calorie rich eatables which are commonly found such as peanut and other nut butters, potatoes, cheese, lean red meat, oils, rice etc and always try to add some fruit in every meal that you take.
Also make it a habit to have at least 5-6 meals daily which should be at regular intervals comprising of calorie rich foods. But remember Your meals should be a combination of foods which are higher in calories, minerals, and vitamins, as opposed to foods higher in fat, salt and sugar.
Protein is a key factor in building muscles so in order to gain muscle mass you need to eat sufficient amount of protein. It is recommended that you should eat high protein food after your workouts.
Weightlifting is another excellent way to gain weight naturally. Initially to get a proper idea about it you can join a gym. It is indeed another important step on how to gain weight, so make sure you are doing it correctly.
Gradually you need to focus more on the free weight exercises that work the large muscle groups. For mass, you can go for compound free weight exercises like squats, bench presses, barbell rows, pull ups and bar dips. So, let's start taking action to gain some muscles!