Before the new/galactic Republic.
Before the new republic (under chancellor Valorum in the fantom menace) there was was the galaxy (well durr!) with all the planets like Geonosis, Coruscant, Tatween and Naboo. But not all planets mastered space travel at the same time. The most likely planets to have mastered it first would have been Geonosis or kamino that we know of. There would have been a massive struggle for power and a great un-balance... Until the first Jedi's rose and joined forces (so the first Jedi council was born) and brought order to the different planets. After years of peace some of the members of the Jedi Order began to disagree with the Jedi's teachings and ways. so they left and became THE FIRST SITH!
After years of conflict between the Sith and the Jedi the Sith (thirsting for power) began to kill each other so losing numbers and allies quickly the Sith went into hiding where they were hunted down by the Jedi Order.
At the fall of the old Republic Pre Vizsla's ancestors (probably the death watch from the planet Mandalor) stole the Darksaber (one of the most powerful Lightsabers).
After a massive war all over the galaxy the new galactic republic was formed with the Jedi as a galactic peace keeping force. The new republic was designed so they everyone got their say and was heard the senate was based on the planet of coruscant where the city was built up to become amuck bigger planet then the original.