You got a Steam key and want to know how to activate it? This guide will help you.
You need to have the Steam software, see this link: http://store.steampowered.com/about/
After launching the software, register or log in Steam software.
Click on 'Games' and 'Activate a key', now, follow the Steam instructions.
In order to activate a game on Origin or Uplay, here are a tutorial:
Download the Origin software on the official website: https://www.origin.com/fra/fr-fr/store/download
Create a new account or log in if you already have an account.
Go to the menu 'Origin' and choose 'Use a product code', and follow the steps of Origin.
See the download page of Uplay: http://uplay.ubi.com/ and download the software.
Connect you with your ID (or create a new account).
Go to the menu 'Parameters', and 'Activate a product' and just follow the steps on the screen.