weight gain

Hello eve ladies! I would just like to know your opinions on how to gain weight! I am underweight and it's honestly hard for me to gain weight. I'm supposed to weight 135-140 but as of now I weigh 127(weighed myself today) and I will have weight on me for a good week and a half and then just drop it out of no where and idk if it's because I don't eat enough or what but I want to be a healthy weight every where.