The Teenage Mutant Ninja Turtles comics are not a part of DC Universe. DC Universe mainly focuses on DC Comics properties. The TMNT comics have their own dedicated publishers and platforms.
The New 52 was a 2011 relaunch of DC Comics' entire line of ongoing monthly superhero comic books. It involved reimagining and rebooting many characters and storylines.
The New 52 was a 2011 relaunch of DC Comics' entire line of ongoing monthly superhero comic books. It introduced new storylines and updated character designs.
I don't have a definite answer. DC has a history of reinventing their storylines, so DC New 25 could be a partial retelling with some modifications or it could be a completely new take. It depends on how they approach it.
It could be really interesting. A good DC fanfic in a new world setting might introduce new versions of our favorite heroes and villains. Maybe it gives a fresh take on the DC universe, like imagining how Superman would be if he grew up in a world with different values.
You can start by finding a reliable online platform or comic book store that stocks DC Comics New 52. Then, familiarize yourself with the reading order and story arcs.