Find the middle C on the piano. On a full-sized piano, this is the fifth C from the lowest note. On a keyboard or miniature piano it may be in a different place.
Play the middle C and harmonize with it. Try to match the note as closely as possible.
Move down the scale harmonizing with each note until it becomes too difficult. This is the low end of your range. Mark it down so you remember.
Return to the middle C and move up the scale, harmonizing with each note. Stop when you get to a note that is too difficult to match and write the note down. This is the high end of your range.
Check the notes that you marked down. This is your singing range. You should be able to sing the notes in your range easily. Try playing through your range again to confirm that you are comfortable singing each note.