Open up Steam and,  at the very top left you will see a number of tabs including 'Games'
Under the Games tab, there is 'Activate Product on Steam'
Work your way through  that window selection and enter you product code.
Cheers :)

Make sure you check your email client for the  mail.  be  sure to check spam folders if you don't see it.

Hi, glad of you to join us. First you need to download and install the Steam Client from this website -> http://store.steampowered.com/about/    And then when if you don't have a steam account you can create one when you have started the steam client and once logged in you go to your library and furthest down to the left you have "add a game" -> Activate a product on steam -> enter key here *which you can see on the email you shall have from purchasing the game and once fully activated you can install the game and enjoy. Hope this helped you.

https://neocoregames.com/en/my-profile/history/orders  (here you can see your order and when you click on the three dots next to where it says "COMPLETED TOTAL 39,99$ or w/e package you bought" it will give you options and one of them is to show game key if you can't see the email that has been sent to you. Have a nice day.

Ty verymuch it worked now, with the knowledge of adding /orders behind :D