Follow the following steps,
1) Sign In here with your google account
2) Click on the link developers.google.com [at bottom left]
3) Click on the link API Console [at bottom left]
4) (if this is your first time) The screen will display 'Start using the Google APIs console' and then click on CREATE PROJECT
5)Below the GOOGLE api logo ,click on the drop down list and select CREATE
6)Enter name for your project and click CREATE PROJECT
7) Now activate which ever service you want => i.e. ON the service then check in I agree... and click ACCEPT
8)Click on the API Access [left side] and here is your required API key
or this link can also help you