THE PILL

okay so my mum got me prescribed to the pill not long ago and the doctor said i am to take the pill when i get my period next which is in a couple days time soooo.... 
i thought i'd ask are there any changes when you go on the pill??? as i have heard you can gain weight and im worried about that because even though i feel comfortable in my own skin, i don't want to go back to weight i was when i was 14