@vel4life A post-apocalyptic world is an interesting topic, yet it is a tricky piece. It will require a lot of technological and theorem research. Since you stated that the protagonists needed to find a way to save Earth, one thing that they should work out first is how did the destruction started? Is there a master villain behind it or it is just humanity that forced the planet to its extinction? Is it willed by higher beings? By knowing how it started, they could probably come up with a way to reverse the event...or at least have an alternative in case the planet is beyond help.
Since it is a sci-fi, watch a lot of movies, shows or anime and read a lot of books of the same genre.
(Though, if I would write this, the group either uncovered an ancient artifact or being or a they have happened to invent a time machine that would aid them in their journey.)
~ Just a passerby...