One of the great things about the Mac is that Apple provide a lot of really useful software for free, along with the operating system. Because these apps are "just there" many people either don't use them or are unaware of some of their more useful features.
One example is QuickTime Player, which can do a few basic useful operations on movie files, in this case "trimming" movies. Trimming involves shortening the movie by removing some of the start and/or end. This is often useful because the start of movies often involves trying to center the subject, get a good focus, etc, and is best removed.
Here's how to trim a movie...
In most cases QuickTime Player will be the default app to open many types of movies, so double-clicking the movie file will open it. If the movie opens in a different app, quit form that app, right-click on the movie file, then go to the "Open With" item and choose "QuickTime Player" from the sub-menu.
Note: If QuickTime Player doesn't appear in the list the movie is probably in a format that program can't open, so this tip won't work.
Once the file is open, trimming is easy. Go to the "Edit" menu and choose "Trim".
A timeline of the movie will appear along the bottom of the movie window.
Drag the left end to where you want the movie to begin.
Drag the right end to where you want the movie to end.
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