(If you’re used to the older iMovie, Apple has a support doc explaining where the interface of iMovie 10 has moved some features from iMovie 2011.)

Basic organizing advice:

1. Make a new “Event” for each film you make. New Events have a default name of the date, but you should change it right away to whatever your film is about.
2. Once you have an Event, you can
   1. import media – i.e., clips – to it and
   2. make a new Movie in it. (Both Movies and Trailers are kinds of “Projects,” which is probably why Apple doesn’t just call “Projects” “Movies.”)