Make sure you are logged in. If you are, your username will be visible in the bottom right corner. Otherwise, use the button labelled "sign in" in the bottom right corner.
Click the save button (third button from the left, at the bottom). If you started with a blank save this will say "[untitled simulation]", otherwise it will have the title of the last save you opened.
Give the save a name and description then click "Save simulation". Remember to check the "Publish?" box if you want other people to be able to see it.