A Site Collection is a container for storing SharePoint sites. In SharePoint Online, you create a Site Collection using the Office 365 Administration interface. Using SharePoint Online is nearly identical to using SharePoint that has been installed at your local organization. The only difference is that Microsoft manages SharePoint in its own data centers in the Online version and you access the application over the Internet.
From a user’s point of view, it’s just SharePoint. At the highest level of administration, there are differences in how you create a Site Collection.
On the administration page, click the Admin drop-down list in the upper-right corner of the page.
Select SharePoint.
On the SharePoint administration page, click the New button.
Select Private Site Collection from the drop-down list.
Fill in the form with the information for your Site Collection.
Click OK.
Click your new Site Collection.
You can view its web address and other information.
Enter the website address in a new browser window.
You can now access your new Site Collection in SharePoint Online.