Gather all the CDs and MP3s that have the songs you want to go into your playlist.
Using a multi-track audio program like GarageBand, Ableton Live, Audacity or Cubase, import the songs into an audio channel in the order you want them to play.
Lay each song or file back-to-back so that they flow into each other.
Once you have all your songs in place, under the file menu choose "Export" and export the entire mix into one audio file.
If your audio program does not export to MP3, find the WAV or AIFF file and convert it into an MP3 using iTunes or any other MP3 encoding music player.
To make the mix available to friends online, choose a suitable podcast hosting service like podbean.com.
To make a mix for your friends using an internet radio service, find a site that has the music you want to mix. Several sites exist that offer a playlist option, like Grooveshark or Songza.
Search their on-demand library of music and add each song to your playlist.
Once you have completed your mix, find the "share" option on the website and email it to all your friends.