I agree that a dietician would be best, but I don't think it's of vital importance. Since you've never had an eating disorder and you're only doing this because you want to have a 'healthier' weight, I don't think you really need any kind of doctor to take you by the hand here.
I have battled eating disorders (mainly anorexia) for about seven years and I have by now learned almost all that one can learn about food and calories, haha! ;) So I'll give you some food ideas that seem to work for me and/or I know are recommended for people who want to gain weight the healthy way.
- healthy milkshakes (fresh milk and low fat yoghurt mixed with fruit and sugar)
- whole-wheat bread
- healthy cereal, as a snack on its own or combined with a dairy product
- soy products
- bananas and other high-fiber fruits
- icecream
- a few extra snack moments a day
- various kinds of nuts
- eating 10-20% more of your main meals of the day
- drinking fruit juice or milk instead of water and tea
There is absolutely no reason to hurry the weight-gain, so choose your own pace. Just try to make sure you get all the nutrition you need and to not lose any more weight. Keep doing what you like to do and try to find what works best for you. You'll be fine :)