College student

i am a college student whom has always had issues with staying with something. I always quit when it gets hard. Including exercise. I also have always been small/skinny (I have NEVER had to watch what I ate) and I finally gained weight last year. But it was fat instead of muscle. So I am trying to eat healthier and exercise as well. I want to lose the fat that I have (not much but I don't like my belly pudge) and gain the weight back in muscle.
But I was also told my the doctor today that I need to eat a more fiber enriched diet and to drink 80 ounces of water a day.