Navigate to your Team Site.

From the ribbon click on + New.

Select App from the dropdown.

In the Site Contents page from the Left Navigation Bar select:
  - Apps You Can Add for a complete list of current apps available to you;
 -  From Your Organization for a list of apps produced in-house;
 -  SharePoint Store for a list of commercially available 3rd party apps.

In you cannot see the app on the page, type a ketword into the Find an App box to search for an app.

Click the app to add it.  You may need to provide information required by the app.

If you are adding an app from the SharePoint stire you may need to provide payment details.