A good place to start would be to start logging your diet over a week.
Keep track of everything you eat, EVERYTHING! You can use fitday to do this. It’s very easy to use.
Once you have recorded you’re macros and you have read all the stickies you should have a good idea of how to go about gaining weight.
Steer clear of weight gainers etc. You need quality protein and carbs. Nothing fancy, tuna, chicken breasts, brown rice, whole-wheat pasta and green veggies (spinach, broccoli) with these basic foods you don’t have to break the bank and you don’t have to spend hours in the kitchen.
Sounds like a lot of work but apply your mind and keep it simple.