If you have to add new apps to SharePoint, you have to first setup an app catalog site. Here are the steps to get it setup.
Open and Login to the Office 365 admin Center Office 365 Admin Center
Open the SharePoint Admin Center by selecting SharePoint under Admin Center.
Select Apps from the left navigation in the SharePoint Admin Center and then select App Catalog.
Create a new app catalog site and then you can begin adding apps to SharePoint.