Hello Shalom
Thank you for your question, it would be helpful to know the reasons why you would like to gain weight as this would help us to provide a more specific answer which is more suited to your situation.
There are however certain key things required to gain weight in any circumstances. Firstly you will need to increase your calorie intake over and above what you are normally eating. So it's is important to track your calorie intake for a while, there are many apps and tools that can help with this, this will also give you a good indication of what foods you are eating on a regular basis and any areas where you may be lacking.
In order to gain weight you will need to eat foods that are calorie and nutrient dense as well as eating more meals. Eating little and often is an easier way of increasing your food intake. Think about foods that are high in good fats so nuts, avocados, coconut oil and salmon. You will also need to increase carbohydrates but again you need to ensure these are good quality complex carbohydrates so brown rice and whole wheat pasta, starchy vegetables and whole fruits.
Rice cakes spread with nut butters or avocado, milkshakes made with nut milks, avocados and bananas with added coconut oil are all foods that are calorie dense and will help to give you the additional calories required to increase weight.
You should also ensure you have good levels of protein so good quality meats, fish and pulses. If you struggle to eat adequate amounts of protein then an easy way to boost your diet and calorie intake is with a protein shake, here you can add in fats and fruits to boost these and shakes are a good way to increase overall calorie intake.
I hope that gives you a few ideas to start with.
Maria