Setting up profile tax options for VAT / GST

What do I select for "Taxes" and "Tax rate" if my prices already include 20% VAT / GST?

You should enable taxes by selecting Taxes: “fixed tax rate” and enter “20” into the field “Tax rate (percent)” below.

What do I select for "Taxes" and "Tax rate" if my prices are entered excluding 20% VAT / GST?

In addition to the above, enter "+20%" as the profile price, which will make WP-Lister add 20% VAT / GST to your prices.

Is there anything else I need to do to make sure my orders will be created with VAT / GST?

Yes, there is. Please check out this article for more details.