Some cellphone camera apps have built-in "trim" in their tools.
Mac (Apple) computers have a free video editing tool called iMovie
Handbrake is a free package that will convert (transcode) your clip to lower resolution (and smaller file size) - there are some key parameters to watch for, in particular "bitrate" (how many bits per second) that determine the size of your finished video file. https://handbrake.fr/