In general your audio collection is much bigger than the storage on your portable allows for.
Often file formats supported by your media player are not supported by your portable.
As portables are limited in space and sound quality, it often don’t make sense to use lossless formats.

One option is to do it manual.
You build a separate library that fits on the portable and has all the audio in e.g. 250 kbs MP3.
Maintaining a second library is a lot of work.

A better one is to let the media player do the job.
You make a playlist, a list of songs you want to have on your portable.
You tell the media player to transcode= converting the audio on the fly, to a format supported by the media player.
On connecting the portable to the media player it will compare what is in the playlist and what is on the portable.
If there are differences it will sync so all what is in the playlist but not on the portable will be transferred (and converted if needed) to the portable and visa versa depending on the settings.