Find the middle "C" note using a properly tuned instrument.
Complete a scale for each octave, which is seven notes long between A to G. This will follow a "Doh-Re-Mi-Sol-La-Ti" pattern, with "Doh" being the lowest and "Ti" being the highest. The scale returns backward -- "Ti-La-Sol-Fa-Mi-Re-Doh" -- to return to the lowest note. The C chromatic scale divides the octave into its semitones.
Sing along with the instrument, gradually increasing an octave every eight notes to reach the higher notes. Stop when the notes become uncomfortable or are no longer natural to sing. This will be around the notes where you switch from your chest voice, which is strong, to your head voice, which is airy. This is the limit of your upper natural range.
Return to middle C and use the same type of scales going to lower octaves in the eight descending notes, matching your voice to each note.
Sing along with the scales to the lowest note you can comfortably sing. This is the lower note in your vocal range.