Calories are key- you need to eat more food than you're burning up in order to put on muscle and add strength. Eat for the body weight you want to be.
Ensure you get all essential nutrients- your health is vitally important. Adding in a multivitamin while also increasing the right type of calories will help your body process and build muscle.
Don’t miss meals- draw up a meal plan and stick to it.
It will take time- expect small gains; consistency will help you achieve your target. Your focus shouldn't be to gain weight or muscle fast. Always focus on the long term.
Consume ~500kcal per day more than you need- log and understand your caloric intake. The macronutrient makeup of the food and drink you take in is important to know. The most common and effective macronutrient ratio for muscle gain is 40/30/30, meaning 40% Carbs, 30% Protein & 30% Fat.
Exercise appropriately- seek credible advice on workout plans. Julie's Strength Training Plan is a perfect primer for muscle and strength gains!