I have recorded music to computer (operating on Windows Vista Home Premium), but in order to save, what is the best file extension to use? MP3 format is common, but I have Windows Media Player on computer, so can I save in .mpa and will cd’s play on car cd player and also playback on computer?
MP3 is a compressed format it throws away audio data in order to achieve a smaller file for use on portable players (so you can get more songs on the player).
For a proper CD you will need to export 16-bit PCM stereo WAVs (part of the Red Book standard for CDs) - and you will need to burn a "music CD and not a “data CD”.
Some modern CD players will play data CDs with a collection of MP3s on them but most will not.