"The Future Begins With You and Me" is a montage song from the Season Eight episode, "Goobacks".
Background
The present is being overrun by poor people from the future, known as Goobacks. They work for cheaper wages, which puts present day people out of work. Stan Marsh suggests cleaning up the environment and making the present time cleaner, as it will benefit the people in the future.
If successful, there will no longer be a demand for them to time travel in search of jobs. During the montage, the future people began to disappear while various townsfolk are seen picking up trash, using electric cars, and planting vegetables.
Lyrics
We've got to work for a better future,
we've got to join hands for tomorrow.
Take the first step and you will see the future begins with you and me.
We can start to make a difference if we want it for our children
Recycle that can and plant that tree,
'cause the future begins with you and me.