Play a note near the middle of what you think your range is. If you are using a piano, start at middle C. Sing the note you have played.
Continue to play notes, moving down the keyboard one note at a time. When you are unable to sing a note clearly, you have reached the bottom of your range.
Return to the first note you sang. This time, work your way up the keyboard until you are unable to sing a note clearly. You have reached the top of your range.
Consult the musical arrangement you are planning to sing. Find the part that is within your range. This part is your vocal part.