In order to activate your Product Key on Steam, first, you need to install the Steam Client Application. You can download it here.

After you download and launch the app, follow these steps:

1. Sign in with your Steam logins, and select ADD A GAME as shown in the picture below. You will get 3 options, from which you need to select the Activate a Product on Steam option.  

   
   

2. After the Product Activation window pops up, select Next.

   
   

3. Select the I Agree option since Steam Subscriber Agreement needs to be agreed upon in order to continue.

   
   

4. Copy the Game Key and paste it in the Product Code field, then click Next.