How do I get gst as the tax on cost of my inventory?

Good afternoon, jkuikm.

Thanks for reaching out to the Community for support. QuickBooks Online provides the option to set up and use sales taxes for your transactions. If you haven't done so already, I suggest taking a look at this article for more detailed information as well as steps on how to proceed. 

Don't hesitate to reach back out if you have any other questions.