If the setting of the story is entirely fictional it is always a good idea to draw a map and several appendices defining species, environment, diseases, temperature, food, currency, languages, physical features, diseases, etcetera. Very little of this material will make it into your book, but you need to have it well established before you start writing.
In the Lunar Chronicles by Marissa Meyer characters exclaim, “Oh, stars!” or “Aces!”
In the Maze Runner characters say, “Good that!” when they agree, and also developed an array of curious swearwords.
In An Italian Adventure, characters swear in Italian, “Oh, cazzo!” or “Merda!”
Who am I to give you advice?
I am the author of The Italian Saga (#TIS) A series of YA novels taking place in Italy. The first three installments of the novels are available as e-books, paperbacks and audiobooks.
The books were produced through my own, small, indie press, Kuki Publishing. I am not working for a big publisher. I do not have a big budget. I simply worked really hard to make my dreams come true and I do whatever I can to help new authors do the same :)
If you want to help me out, check out my books here, reblog my posts, and if you read any of my novels, take a minute to write a review <3