ffmpeg is not
avconv – there's a difference between those. Many simple commands will just work fine in either tool, but you should always download a recent
ffmpeg version instead of doing an
apt-get install ffmpeg, because you won't get the "real" ffmpeg.
Your command is quite off. Have you had a look at any examples or the documentation? We also have a blog post on ffmpeg.
You first have to specify an input file with
-i <file>, then the encoding parameters, and then the output file:
ffmpeg -i old_video.mp4 -c:v wmv1 new_video.avi
That's the most basic command to achieve what you want. The audio will be converted to MP3 here using the
libmp3lame encoder. Choose a bitrate by setting
-b:v 500k or whatever you need. Same for audio with
-b:a 128k or similar.