where is iMovie located

Press on Finder > Search for iMovie in your Applications folder > Press on the purple icon, with a video camera in it, as shown to the right. When opened it, press on the 'plus' button to start a new project in iMovie. 

The iMovie app is located in the Applications folder. The iMovie library is located in the Movies folder (Go/Home/Library) by default, unless you move it somewhere else.

—Rich