You haven't given us enough information. It all depends on what kind of mp3 player you have. For some of them you can simply mount the player as an external drive and copy the files to and from it. Others, such as the iPod, have non-standard upload requirements and need special syncing programs in order to get playable files.
As for "essential" software, it depends on what you want to use the system for. Everyone has different ideas of what essential means.
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