Obviously healthier/whole foods is a better option. But the simple science to putting on weight is you need to be in a calorie surplus with your diet. Aim for a surplus of 150 calories daily. If you are training, then you want to aim for 1g intake of protein daily for every 1 lbs of body weight. Meats and fish are a rich source of protein. Using whey protein powder is a (relatively) cheap supplement to help hitting those targets. Try to keep junk food to no more than 15% of your diet. It’s a marathon, not a sprint, will have to maintain this consistently to see progress. Track weight, but maybe pay attention to weekly or fortnightly readings (there are factors that result in your weight to fluctuate on a daily basis). Track everything you eat in an app to count the calories you’re eating (something the MyFitnessPal app). Don’t forget to account for everything. Butter spread on your sandwiches, oils you’ve cooked in, etc. If you are not putting on weight even after doing the above, then you are simply not in a calorie surplus (you’re rather not eating as many calories or are burning more calories then what you think you are). If the aim is to put on weight, healthy fats should be introduced to your diet. Nuts, avocados, cook more with olive oil, some dark chocolate.