@OP: Look up the timeline on wookieepedia.
In a nutshell...
It won't really start to get interesting until you reach around 5000 BBY. The Republic started in 25,000 BBY. Before that (as you will learn in KOTOR) the Infinite Empire was around before that.
Prior to the 5000 BBY mark (or maybe my memory is rusty), well there was the first great schism and exiling out dark jedi who went to the sith system and found a desert moon there and some apparently indigenous people (actually once hailed from Droumund Kaas). They were deemed sith people. The dark jedi proclaimed themselves lords of these sith people, interbreeding with them. Eventually this culminated into the Great Hyperspace War.
After that everything is history. About a thousand years later Exar Kun did his thing. Decades later Revan and the Exile. 3 centuries after that the resurgent sith empire emerged.
As best as I can determine, the galaxy after that point descended into an endless war of stalemates that would span milennia, despite whatever specifics. At one point the republic was even thought to have ceased to exist on any official level.
Later about 1000 BBY, A young man, Dessel, seeing only injustices and wrongdoings by the Republic joins the Brotherhood of Darkness. This leads him down the path that would make him Darth Bane. Orchestrating the Bortherhood's demise, he commits the sith order to secrecy with the rule of two.
A thousand years later the films.
Then 135 years after ROTJ ended is the Legacy series.
That's pretty much it.