Set total cart amout If the user didn't buy anything ever, at the first time he gets a 20% tax of total shopping cart amount.
I'm newbie in Drupal and can't understand how to implement it.
Say I'll do checking for first order (I imagine how), but I can't find how to change total cart amount without changing every item price.

I hope you can help me :)
The trick to this is to create a discount. Force users to register before purchasing an item and than check the order history of a user at checkout. If the user has no order history at checkout than give him a 20% discount, if not than charge retail price.

This can be achieved with 


http://drupal.org/project/commerce_checkout_login
And either a custom module or rules