You are tiny, but for your height you should probably around 100 pounds or so. I wouldn't suggest fast food and sweets like some of the others are telling you. They'll only make you tired, give you pimples
and will skyrocket your cholesterol
. I would suggest however to eat 5 good sized meals a day with alot of protein (beef, chicken, seafood etc) and carbs (bread, pasta, pretzles). Salads with full fat dressing and tuna salad with full fat mayo are good snack options along with some frozen foods like hot pockets, pizza and so on. Sweets are ok, but in moderation like any other "diet" - you don't want to overload on sugar. Light weight training
will help you put on muscle also and weights more than fat. I wouldn't suggest cardio though if you want to gain weight. Good Luck sweetie!