Need to Gain Weight

Any tips on how to gain weight? I want to get to a healthier weight before conception if possible.

I feel like I eat and eat and eat and do not gain anything it's so frustrating! I'm afraid to exercise because Im afraid of losing weight.

Ive been small since I was just a girl. I currently am 112lbs and am 5'6". I eat breakfast, lunch and dinner, I just started having an after dinner healthy snack (apple w/almond butter). I enjoy eating and eating healthy and make healthy fertility friendly foods typically I just can't seem to gain any weight!