Pick something you want to do then google it.
For example, I batch recorded a bunch of videos. I wanted to slice them up with ffmpeg before I brought them into the video editor. To do this, I use the command
ffmpeg.exe -ss 00:00:05.00 -i input.MP4 -t 00:00:18.00 -acodec copy -vcodec copy output.mp4
-ss is the start point
-t is the offset from that start point
-acodec copy copies the audio codec
-i is my original video file
-vcodec copy copies the video codec