For the first Community Playtest you've got your Steam key, your hardware meets the requirements, now you need to get the game downloaded and installed!
Step 1:
Open Steam and head to your Library.
Step 2:
In case you haven't redeemed your Steam key yet, you'll want to go to the Games menu at the top and select 'Activate a Product on Steam...'
Once you've redeemed your Steam key the game will appear on the left side where you can click on its name. Be sure that your Steam account is linked to your 2K Account.
Step 3:
The big friendly blue 'INSTALL' button is your next goal. Choose where it needs to go and click 'Install' again.
That's it. Give it a little time to finish and then you are good to go!