How can i gain weight? Please help?

I really didn't care about gaining weight until this mean energetic girl in my school said " real women should have curves" and she told me only dudes should be skinny. She does have a nice body to be honest and i'm kinda jealous, because she have nice curves, nice round , and big ****, she's tall like 5'7 and weights like 140 pounds, well i would just say she's healthy, anyways she always tries to make me jealous by saying " all the hot guys stick their d*cks in her "nice" *** and thouch her big " soft" and her curvey thighs this is what she always says, and she also says that she has alot of very hot/y athletic boyfriends and they all don't like girls with skin and bones. sometimes outside the school she kisses all her boyfriends at the same time infront of me well i'm just getting sick of her. I'm 5'3 and weights 85 pounds, and i always feel tired and don't have the energy to do activities in gym like this girl. I'm glad i typed everything about her and down and got the anger out. How can i gain weight and how much should i gain. Thanks i'm 14 and this girl is 14 too and does all of this nasty stuff.