Canonically, Nero started in the OG/main timeline, in the 24th century (Some time after DS9/Voyager, or the movies with Picard like nemisis.) He was the captain of a mining ship, then there was a star that went kaboom; and destroyed the romulan homeworld.

Old Spock was working as an ambassador and was bringing Magic Red Stuff™ to stop the supernova by creating a black hole or something. In any case, they arrived way too late, Romulus was destroyed anyway, the supernova was stopped (not how physics works, but okay,); and the Jellyfish (Old-Spock’s ship) and the Narada (Nero’s ship) were sucked into a black hole. the black hole sent them back in time; the Narada to an earlier point, Old-Spock to a not-so-recent timeline.

The USS Kelvin was destroyed after Spock came back, responding to a distress single and getting the shit kicked out it. the Kelvin, incidentally, was commanded by George Kirk, James T. Kirk’s father. (in the OG timeline, George was present in Kirk’s life, etc. Which completely changed Kirk’s nature.)

Nero’s and Spock’s presence irrevocably altered that timeline, creating a different universe. Because the Kelvin was where things started diverging significantly, and because Fans are fairly simple minded… (sorry.) … this timeline is now called the ‘Kelvin timeline’

Out of universe, Abrams found that the OG lore background was far to restricting, and didn’t allow him his much-abused lenses-flares. so he threw a tantrum and created a new timeline that would allow him his, “creative” liberties.