Steve's advice is top drawer, but here are some simple steps:
1. Shoot your video with your green screen directly behind your main subject.
2. Place your video on the timeline. (Usually I put videos in the Project Assets folder first and then drag it to the timeline. Why? Easier to organize my videos and other media.)
3. Select the video on the timeline and select FX from the Edit bar--don't select the FX-pencil; just the FX icon.
4. From the Video Effects menu choose "Keying", and "Non Red Key" from the Keying submenu (...don't get me started on this label name....)
5. Drag the Non Red Key on top of the video on your timeline. In the Applied Effect menu that appears, select the Defringing drop-down menu and choose "Green" (the default is None).
That's it for setting up PRE with Green screen. Your green objects should now be fully transparent. For this to work optimally, you will need to have even light on your green screen, and all all elements you want to appear behind the main subject must be below the video on the timeline. Use the Cutoff and Threshold sliders to make fine adjustment, but an even light on your green is your most helpful tool.
Extra Tip: If your effect is going to be used on your entire video, be sure to place the effect on the timeline before you start editing