How do I become healthier and gain 25 pounds?

I suffer from Fibromyalgia. I read a book called "The Candida Control Diet". I want additional tips on how to gain weight because eating healthy sometimes makes me too skinny. I only weigh 100Ibs and I'm 5'6". I don't believe in using medication but I can't afford to buy homeopathic remedies.
I want to gain weight WHILE EATING HEALTHY.