The best thing to do is to do it slowly. If you're doing it for your health I think that the best option would be to actually start eating some starchy foods and doing non-cardiac exercise (like weight lifting.) This will help to add muscle mass, which is heavier and healthier than fat. If you just THINK that you need to gain weight to be healthy, if you can run a mile you're probably in fine health and just let your body do what it needs to do.

If you want to gain weight for appearance, it may not be worth all of the bulking up that you'll have to do. If your body is happy where it is it might make you LESS healthy to gain weight, especially if it's just to look a bit better in your bikini. I'm sure you look great the way you are!

In short, everyones bodies are different. I personally gain weight like crazy on biscuits, but can eat candy bars and LOSE weight. My mum packs on the pounds when she eats oatmeal. There's no sure fire thing to cut/include in your diet to lose/gain weight. Just make sure you aren't only eating cupcakes or anything, because that's just going to make you sick and grouchy. Good luck!