Shopify is not able to inform customers what their tax responsibilities are, and if you are unsure, we would always recommend contacting a tax professional to gain more information about when you need to charge sales taxes on your orders.
Generally, merchants should first determine what the sales tax laws are for the country or region they are based in and if and when they must register for sales tax. These rules vary on a country-by-country basis and can also vary for different regions and areas within a country too. As you are based in Hong Kong, you'll first want to look into the laws and regulations for charging sales taxes for sales made within Hong Kong itself. I did a quick search online and came across this document from Deloitte that seems like a good starting point (note: as this is a third-party document I cannot verify it's accuracy, so doing your own research is encouraged).
If you want to sell internationally, you'll also want to do your research regarding selling into other countries and how these countries' tax laws will apply to you. For example, if you're selling from Hong Kong to the United States you may not need to charge any sales tax to US-based customers if you have no presence in the country, but this can change if you make a certain amount of sales or bring in revenues over state-determined thresholds.
I hope this has helped but please let me know if you have any further questions.
Victor | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog