To open an account click on this Link and follow the necessary steps to create your AdMob account and obtain an AdMob Key.
What you need to start with is a google account (i.e. my email@example.com).
Where to find your AdMob key
To find your AdMob Key, log into your AdMob account and click on the Monetize tab on the top.
Then click the Monetize new app button.
Click on Add your app manually and select the platform (Android or iOS).
Click on the Banner button.
Add the Unit name.
And finally collect your AdMob key as shown in the red highlighted box (barred part)