I have read that using use-package, one can manage installation of the packages from the init.el file. Is there any specific command for that ?

Background: I'm planning to port my existing init.el to use use-package. But I'm not able to find any documentation of how to install all the dependencies of my init.el file. I can see in the documentation that they have the :ensure keyword which will install the software if it isn't present already, but it raises few other questions:

1. How to make use-package understand that you have to install this package from MELPA and that from marmalade.

2. Is there a single command for installing all the packages from init.el.