It's not complicated.

1. First go to the accounting section of your Odoo back-end. Scroll-down until you see the configuration section (left-menu).
2. Click on the taxes.
3. There will be a sub-menu below (folded open), click on the first one (called taxes).
4. then your system will show all the tax-rates in your system. Choose the one which is used in the webshop (sale-tax in my case 21%),
5. Select that rule.
6. Now you will see a lot of details of that specific tax rule.
7. click the edit button, and then tick the box (tax is included in Price). Do this for all your tax rates which are used, for selling your products/services. 
8. Click on the save button.
   
   your done!
   
   Please give my a thumb up vote, because my karma was to low to add a screenshot of it, to make it much more understandable.