Office 365 has lots of inbuilt apps with it and If you want to access Microsoft Office Sharepoint Online then you need to first go on to this link
http://portal.office.com/ then on the login page ad your credentials.
1. After successful login, there are two ways to move into Sharepoint Online, first way: please click on the left side 9 Dots rectangle box.
2. Then click on All Apps to extend all the Microsoft Office 365 Online Apps.
3. Then Search or Scroll down and search Office 365 Sharepoint Online App.
4. One another way to login into Microsoft Office 365 SharePoint Online is by Click All apps from the Home page after successful login
5. Then find Sharepoint App and Click on the Sharepoint Logo or Name
6. After clicking on Sharepoint Logo you will be forwarded it to the Sharepoint Online Homepage.