If you already have a steam account then open your library and click on the button in the bottom left that says "add a game..." and choose "Activate a Product on Steam...". Click "Next" then "I Agree" and enter the code that you received in the email. There will probably be more options after that but I can't see them without a key to add, however they should be self explanatory.
If you don't already have a steam account you can create one at https://store.steampowered.com/login/ and then install steam via the little green button in the top right of that page.
