On the lower half portion of the big circle, draw a smaller circle.

Summary:
Draw a big circle. Draw a curve line that will serve as the outline of the helmet. Draw a slanted "A" on the right side of the drawing. Draw a vertical line at the tip of the lower side of the "A" and connect that line with the upper line of the "A" using a curved line. Outline the drawing to complete the details of a face mask. Outline the shell of the helmet. Add designs as desired. Colour accordingly.