Sing a note that is comfortable for you to sing. Identify the note on the piano using trial and error. Sopranos should start on a C, mezzos should start on an A, tenors should start on middle C and basses should start on A in the bass clef.
Following the piano, play and pitch match ascending half-steps from the note you started on in Step 1. Sing until you have hit the highest note that is comfortable in your range. Write this note down. For a soprano, the highest note should be between A and C above the staff. A mezzo should end between an F and an A on top of the staff. The tenors and basses should sing the same pitches respectively an octave lower.
Begin again in the middle of your register using the note from Step 1, and sing until you have hit the lowest note you can sing comfortably. Write this note down.
Add a fourth to the lowest note and subtract a fourth from the highest note. This will give you your octave register that is most comfortable for your voice. For instance, if the highest note you could sing ends up as an F, then the comfortable range of your voice ends at C.