You should get a message that you need to create credentials. Click the "Go to Credentials" button.
Give the key any name you like. (You can leave the "allowed referers" box blank, but don't give out your key to other people.) Click "Create API key" and your key will appear. It 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 and then use the "Credentials" link under "APIs & Services" in the left sidebar.