Where and which Api key should I get for Google Maps

Hello All,
I’m really new so be kind with me please :D.

I’m trying to build an app that in the first screen check if Position sensor is enabled and if Gps position is fixed. After that I would like to put a button to start the GPS tracking (in background) and app will open a second screen. Here in the second screen, I would like to have a visual map with actual position of the user and a button to record that position (Lat/long) as a string.

I would like to know wich APi key should i get from the google api control panel to reach my goal.

Thanks for your help.