Its becoming longer and longer and I've clicked and read everything listed.
Up in the top right corner, near your username, where it has the number of unread notifications, click on the arrow next to it. That will open a drop box. Scroll all the way to the bottom, it should say something like "Show all notifications" Click on that. A screen will open up with all the notifications. In the top bar there should be a box you can click that will check off all the notifications, or you can click the box on each of them. Then, at the bottom, there should be an option to delete.
I don't have any notifications at the moment so I can get the steps exact, but it's something like that.