What diet can i use to gain 15-20 lbs?

I am 5 foot maybe an inch, and 80 something lbs, so to avoid going to a hospital, I'd like to help myself with your help. What kind of plan can i go on with exercise so i don't look like gelatin, that will allow me to gain 15 to 20 lbs?? And how long until i can see some results? I would like to stay fit! Detailed answer are appreciated!! Thanks :)