Check https://www.avalara.com/. You give them a list of products and/or services, the location of the buyer, and they tell you the taxable amounts for each item. It's all via API so can be built into your cart and checkout easily.
You call an accountant and find out. I pay 30$ per month, someone in another country may pay $300-3000+ per month. It also greatly depends on your revenue and other factors.