Click 'Create project' and give it any name you want.
Click on the 'Credentials' link in the sidebar. Under 'Public API access', click 'Create new Key' then 'Browser key', then 'Create'. (You can leave the 'allowed referers' box blank.)
Your key will be 39 characters long and will probably start with 'AI'.
If you need to retrieve your key again in the future (or create additional keys), just go back to the Google Developers Console, click on your project's name, and then use the 'Credentials' link under 'APIs & auth' in the left sidebar.