When you complete a course (i.e. your progress shows 100%) you will be able to download a completion certificate for that course. Course certificates can be found in your profile: access your profile via the 'Profile' option on the main menu.

Additionally, completion of some modules within particular courses gives you access to a certificate. Module certificates are accessed from within the course they relate to: you will find them (where available) at the bottom of the course overview screen

The following video shows you where you can access both course and module certificates: