The Receipt / Invoice templates will show taxes charged on orders automatically. It is Shopify that calculate the tax amounts, based on your Shopify tax settings. Our documents automatically display the tax and tax rate used.
Check Shopify admin tax settings
If you are not seeing taxes in your invoice, double check your tax settings in the Shopify admin. You can do that on this page: https://shopify.com/admin/settings/taxes
Once taxes are setup in the Shopify admin, then they will automatically display during checkout, in emails and in the invoices.
Important
Shopify will not update old orders when you make tax changes, so only new orders will show the new taxes you have setup.