To be able to play an MP3 file or files, you will need a MP3 Player.  Many of these exist on the internet, and even one being an updated Windows 98 Media Player (version 6.0), available for download off the registered windows update site,  http://windowsupdate.microsoft.com (NOTE:  This site is only accessable via Internet Explorer 4.0 or higher and Windows 98 MUST be installed).  As for my own oppinion, the most highly used MP3 player is WinAMP, avaible from Nullsoft's home page at http://www.winamp.com.  This player, based on Nullsoft's Nitrane MP3 decoding engine allows users to create playlists and other advanced features to build a collection of their favorite music.

If you do not wish to spend time on the internet, looking for free downloadable songs, you can create an MP3 collection of your own using your store bought CD's.  This allows you to mix songs from various artists without having to swap CD's.  Two excellent MP3 encoders are Xing's Audio Catalyst which is an encoder and ripper in one, and Xing's MP3 Encoder which is a faster, better encoder, but requires you to have previously ripped the song from the CD.