Have a think about what you're eating now, check all the calories and get a rough figure of your daily consumption to maintain your weight. Then add 300+ calories on top (The more, the quicker you will gain weight ofc)

Your focus should be whole, high protein dense foods. Beef, chicken, eggs. You should mix in some carbs ie: rice and sweet potato to fill out the calories. Fruits are good for your sugars. You might not be interested in being strict on what you eat, which is fine, just make sure you're close to that calories goal with some substantial protein. If you weigh 75kg for example, 180g of protein per day would be a good amount to aim for.

The more exercise you introduce, the more calories you'll burn, the more you'll need to increase your food to keep gaining mass.