the input should already be set to the internal microphone
now click the round button with the red dot in it to record your voice
click that button again (now it has a black square in it) to stop the recording once you finished what you wanted to record
then go to FILE and click SAVE AS and save it where you want
Sadly QT Player is not smart enough to save it as a .wav or .aif or .mp3 or any other audio file format, but saves it as QuickTime movie (.mov), thus you can use iTunes to convert it to a proper format.
To do that
open the .mov in iTunes (add it to the library)
go to iTunes Preferences (CMD+,)
then go to the GENERAL section in there
then click IMPORT SETTINGS at the bottom
under IMPORT USING select the appropriate format
(WAV and AIFF encoder are lossless and take about 8 MB for one minute of audio,
AAC and MP3 encoder are compressed formats, thus you can have, with the right settings, a file, that takes 1 MB per minute of audio