How are the seasons formed?
Due to Earth's Axial tilt.
Earth has a 23.5 degrees axial tilt which causes seasons. An image would explain it better.
Lets take one Hemisphere as a reference.
The direction of the tilt never changes it always points in the same direction. In June as the figure shows the Northern Hemispheres is directly facing the Sun which means that there is Summer in the Northern Hemisphere.
Six months later in December, the Northern hemisphere is tilted away from the Sun and it's Winter in the Northern Hemisphere.
In March both Northern and Southern Hemispheres sweep equal portions of Sun and its Spring in the Northern Hemisphere.
Six months later in September, it's Autumn in the Northern Hemisphere.