If you do not have Steam installed yet it can be downloaded from this page: http://store.steampowered.com/about/. Locate the green button labelled "Install Steam Now" and click it, it should look similar to the image above. Once the download is finished click the downloaded file to run the Steam installation program. Follow any prompts during installation. Once installation is complete please continue with the article.

Find Steam in your computer's program menu and click it to launch it if it is not already running. You should get the login prompt if you have not enabled auto login. If you haven't created an Steam account yet click the "CREATE NEW ACCOUNT" button and follow the prompts for creating a new account before continuing. Enter your login credentials (account name and password) into the prompt. See image above for reference.

After you have entered your credentials click the "LOGIN" button to continue. If you receive errors after clicking the "LOGIN" button ensure you have entered your Account Name and Password correctly. If you continue to have login problems please contact Steam support. After successfully logging in please continue to the next step.

After logging in Steam will start and may present you with some store offers, close any dialog windows that open before continuing. Find the "Games" menu along the top of the Steam window and click it. See image above.

After clicking the "Games" menu a dropdown will appear, find and click the option that says "Activate a Product on Steam...". See image above.

After clicking "Activate a Product on Steam..." from Step 3, you will be presented with the Product Activation dialog, click the "NEXT >" button to continue. See image above.

You will be presented with the Steam Subscriber Agreement, read as necessary and when you are ready to continue click the "I AGREE" button. See image above.

You can now enter the activation code that was emailed to you after your purchase. Enter it into the Product Code box as shown above. Do not enter the code presented here, enter the one that was emailed to you.

After you have entered your activation code/key click the "NEXT >" button to continue the process.

You will then be presented with the Product Activation Successful dialog. It will list the name of the product that was activated. If your product fails to activate be sure you have entered the code correctly. For example try substituting an "8" instead of a "B" or the letter "O" instead of a zero, etc. You can view this article for more help with an unsuccessful activation. Click the "FINISH" button to continue. See above image.