In the Legends continuity, we first see a lightsaber in *[Star Wars: Dawn of the Jedi: Force Storm][1]*, which takes place in 25,793 BBY.

[![enter image description here][2]][2]

In regular canon, the first lightsabers we see are in *The Phantom Menace.*

[![enter image description here][3]][3]


  [1]: http://starwars.wikia.com/wiki/Star_Wars:_Dawn_of_the_Jedi:_Force_Storm
  [2]: https://i.sstatic.net/DSSki.jpg
  [3]: https://i.sstatic.net/BZB0z.jpg