Why seasons happen
Earth circles the Sun once a year with its axis tilted 23.4°. Here it is shown at four points of the orbit: the two solstices and the two equinoxes.
The tilt never changes direction
Look at the white axes with their red north ends: all four lean the same way. The axis keeps pointing at the same spot in space (near Polaris) all year.
June: north leans in
In June the North Pole leans toward the Sun. Sunlight hits the Northern Hemisphere more directly (overhead at the Tropic of Cancer) and days there are long, so it is summer in the north and winter in the south.
December: north leans away
Half an orbit later the axis still points the same way, so now the North Pole leans away. Sunlight strikes the north at a low, spread-out angle with short days: northern winter, southern summer.
The equinoxes
In March and September the tilt points sideways relative to the Sun. The Sun is overhead at the equator and both hemispheres get equal light and roughly 12-hour days.
It is not the distance
Earth is actually closest to the Sun in early January, during northern winter, and farthest in July. The distance varies by only about 3%, and the two hemispheres have opposite seasons at the same moment, so the tilt, not the distance, makes the seasons.