Gain weigh fast

Hey ladies, well I have never post anything here. Recently I dont like my body I feel so ashame of myself. I weight around 80lbs I'm 19 years old and 5"1. I tried vitamins that my doctor gave me. My boyfriend told me to try and gain weight because he doesn't want me to get sick and it would be difficult when ever we want babies. Do you have any advice to do? I started working out again a week ago not too much to loose more weight but my doctor said to exercise ... I would really appreciate if any of you could help👏 thank you