From your store's Shopify admin: Settings » General » Store Currency.
Customers use this currency to buy your products. It can only be changed before you've made your first sale. If you've already made your first sale, so you need to contact Shopify's support if you want to change your currency.
You should make sure you also customize the same setting for the various payment providers you're using, having your store currency set to USD while getting payments via PayPal in EUR might generate unwanted conflicts.