Honestly It really depends on your metabolism and genetics. I hate to sound like I’m bragging but…I was born skinny and that’s the way I will stay til I die as witnessed through generations of my family.
I eat nutella sandwich for breakfast and maybe cereal. For lunch lately, I just ate hot dogs and hamburger or subway since I don’t cook most of the time so I buy fast food. For dinner I just go through the fridge finding any leftovers or shyt. If I can’t find anything good in the fridge I go out again and buy fast food. I only make legit food if I’m in the mood or for special occasions. I don’t excersize, i just don’t have time with the shytload of studying I have to do for college.
But if you gain weight easily, you should definitely excersize a lot. I suggest running a mile or two each day without stopping, do that for a few months and you should tone down a little. Do crunches for belly fat.