What was Darth Revan’s life before his evil ways?

When it comes to Star Wars you kinda need to say if it's "canon" (post-Disney) or "Legends" (pre-Disney) seeing as Disney made a lot of changes and also removed things from the story.

Revan is a victim of this Disneyfication, seeing as his story got mostly cut. In the new canon he is only mentioned as a side note in Star Wars: The Rise of Skywalker: The Visual Dictionary.

In Legends his story is much more elaborate where he, like many other Jedi, entered the order at a young age and underwent the standard training. Where he differentiates from the other Jedi (the Jedi order) is when the Mandalorians began an invasion of The Republic. The Jedi Council opted to stay out of the war while Revan decided to support the Republic fiercely. This caused him to leave the Jedi Order and take several other Jedi, like Malak, with him.

This made him a war hero to the Republic (Much like Anakin was thanks to Palpatine's manipulations).

Eventually he defeated Mandalore in single combat, but with his dying breath Mandalore revealed that he was manipulated into the war by a Sith. After the war with the Mandalorians was basically won he took his Jedi/Republic forces in to the unknown regions to hunt down the remaining Mandalorians who hadn't surrendered to the Republic. This however was a lie, they instead hunted the Sith.

Eventually they discovered a Sith Empire that was presumed to be destroyed a long time ago. In Revan's arrogance he believed that he and his forces were more then able to take out the Emperor and his forces and attacked without informing the Jedi or the Republic of the threat. But, instead of fighting Revan, the Emperor used his powers to turn him to the Dark Side and sent his forces back to attack the Republic.

For more information on Revan I would advise to read The Knights of the Old Republic comics. and play the Knights Of the Old Republic game.