If you live in a country with multiple tax rates or multiple currencies, you can enable support for that under Setup » Basic » Payments.
Check the "Custom tax rates" or "Multiple Currencies" box.
From now on when you edit a resource, plan, service or add a one time charge you can choose the tax rate and/or currency. Of course your invoices will reflect the updated tax rates, too!
Important note:
All new tax rates or currencies will only apply to NEW invoices generated from the time the change takes place.
Next Step
Let's move on to analytics.