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'.
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 "API Manager" in the left sidebar.
If you needed a key to "fix" a GPS Visualizer map that no longer works without one, open your map in a text editor and look for var google_api_key = ''; and insert your key (with NO spaces) between the quotes. (Also, search for file:// and replace it with xxxx:// if you find it.)