need to convert an mp4 movie to an amv file for mp3 player with video storage

I recently got a crappy little mp3 (64GB if that may be important) player for school and im trying to convert a movie (mp4) file into an amv file to do so, i cant find any websites that dont have some weird file size limit or any programs that arent paid subscription, I'm super novice with techy stuff and could use some advice.