sphere(d = 5); sphere(r = 10); *centered around the z-axis by default
Quick Notes:
Numbers in OpenSCAD are in millimeters. So circle(5); or circle(r=5); draws a circle with a radius of 5mm. When modeling in small sizes, shapes will have a lower resolution (rougher texture).
When you are finished describing your shape, finish it with a semicolon. Objects end in a semicolon ';'.