We have seasons because the earth is tilted (wonky) as it makes its yearly journey around the sun. The Earth’s axis is tilted at an angle of 23.5 degrees. This means that the Earth is always “pointing” to one side as it goes around the Sun. So, sometimes the Sun is in the direction that the Earth is pointing, but not at other times. The varying amounts of sunlight around the Earth during the year creates the seasons.
The tilt of the Earth’s AXIS is the most important reason why seasons occur. We have hot summers and cold winters because of the tilt of the Earth’s axis. The tilt of the Earth means the Earth will lean towards the Sun (Summer) or lean away from the Sun (Winter) 6 months later. In between these, Spring and Autumn will occur. When it is winter in the northern hemisphere, it is summer in the southern hemisphere and visa versa. This is why Australians can celebrate Christmas on the beach as it is in the middle of their summer.
At the spring and autumn equinox, the sun is midway between it’s highest and lowest points in the sky. This usually occurs around 22nd March and 21st September, and both day and night are 12 hours long. Seasons become more extreme the further you go from the equator. A good video can be seen here.