Once you visit World of Tanks main website (worldoftanks.com), you should be automatically redirected to the European (American) server. But it also happens that you stay on the default server - just click then on the globe icon in the top-left corner and select Europe (North America). The top menu bar can be also used to switch the language (which can make site navigation easier).
Then look for option "PLAY FOR FREE", fill all fields and follow instructions on screen. If you have any invite code then check the proper option and type it in a new field. After creation of an account you have to confirm its authenticity (check for an e-mail and follow instructions).
The final step is to download game client: select Game tab and then Download Game. Now you have to make sure that you're downloading a proper version (for European/North American region). After logging in the game you can start your adventure with tanks.
At this point we should mention also a method to activate bonus codes. It can be done from the browser: just login into server (top-right corner) and then click on your account and select Enter bonus code option. After a moment codes will be activate in the game.