AdMob is a service linked to a Google account as well as the AdSense and AdWords services.
This will allow you to get banners displayed in your Android application (game) exported with Intel XDK CrossWalk.
This requires to fiddle with several websites and tools.
Moreover, it won't simply work with your game on preview from Construct 2, you need to publish on Google Play store itself at least in Alpha stage (closed Alpha allows you to simply "publish" a test application and only allow a limited number of google accounts/linked devices to have access to it, perfect for our current need).
The first time you have to set up adverts for your game, the whole process might be a bit tedious, especially since you will be required to read through licensing terms for services before registering to them, and you definitely should do so.
Moreover, there is a cost involved, as you need to own a Google Android Developer account to get access to the Google play Developer Console.
This tutorial will require you to own at least a valid personal license.
Construct 2 free edition is lacking the export features and the licensing terms of the free edition do not allow you to make any money from works made with it anyway.
Adding advertisements to your game made with the Free edition simply goes against the licensing terms of the free edition and shouldn't be attempted.