It should be on your Dockbar at the bottom. Otherwise you can get to Spotlight by clicking on the rightmost icon on the top bar or by using key combination of apple+space. Then type iMovie in the spotlight search bar and you should see it.
well if its not at the bottom where all the other stuff is just click on macintosh and there should be like a little search bar on the corner and just type in imovie if u dont find it there u can always download it of the apple website for free