You need software and a microphone. You have the built-in mic already. If you have Garage Band then you have the software. Details of how to use GB depend of which version you have. As you have a G4 maybe your GB is not current? But basically you make a track them hit the "record" button the speak then click stop. You can trip the recording, adjust the volume then same as an MP3 file which you attach to e-mail.
If you get a better mic results can actually sound professional. Don't expect that with the built-in in mic but it should be good enough for your purpose.