By default, the currency displayed on the site is USD, and I'd like to change it to my own country currency, which is MYR.
This is how I changed:
1. Go to Administration > Configuration > Currencies
2. If your currency is already in the list, just publish or mark it as primary store currency, otherwise click on Add New button to add a new currency.
3. This is the setting I created for my own country currency.
4. As we do not provide the currency exchange feature, so I just un-publish the other currencies, but just published this newly created currency.