How do I add Commentary and reduce background noise in iMovie

You can add a commentary with the voiceover feature of iMovie 10. Here's a step by step tutorial that describes the process:

https://help.apple.com/imovie/mac/10.1/#/mov44e3a4427

The voiceover clip will appear below the video clip where you placed the playhead, and will automatically lower the volume of the clip above it. I assume that is what you meant by background noise. If it's other background noise you are talking about, like hissing or crackling, you can select the clip and click on the little loudspeaker icon in the tool bar at the upper right of your screen and select audio effects. You will then have options to lower the pitch or make other adjustments to the audio.

-- Rich