Start your reading from here:
http://www.uesp.net/wiki/Lore:Tiber_Septim
There are two versions of his story. One is Orthodox and widely accepted by the people of Tamriel. The other is the Heretical version:
Orthodoxy
He was born in Atmora under the name Talos, meaning Stormcrown in the old Ehlnofey. He spent his youth in Skyrim, where he learned the ancient art of the Tongues as well as the strategies of war from the chieftains there. At the age of 20, he led the invasion of Old Hrol'dan, taking it back from the Witchmen of High Rock. Soon after, growing storms made it known that the Greybeards were about to speak. As nearby villages were being evacuated, Talos traveled up the mountains the Greybeards called home, where they removed their gags and spoke his name, causing the World to shake. The Greybeards told Talos that he would come to rule Tamriel, but he must go south to do so.
Talos became General of the Colovian Estates under King (and later Emperor) Cuhlecain. General Talos met the combined army of Skyrim and High Rock on the field of Sancre Tor, who were hostile to Cuhlecain. The Skyrim army joined him when they saw him use the thu'um and knew him to be the son of Skyrim and heir to the Empires of Men. But the Bretons saw things differently. The general was almost killed in 2E 854 by a High Rock nightblade who managed to slit his throat and kill Emperor Cuhlecain. Although he could no longer use the Voice, he went on to command Tamriel with a whisper. He took the Cyrodilic name Tiber Septim, as well as "Ysmir, the Dragon of the North", the Nordic Name of Kings (also called more simply "Ysmir, Dragon of the North"). Tiber Septim died in 3E 38.
Heresy
(This alternate story shares many of the details and events of the official version, but differs in key aspects.) Hjalti Early-Beard was born on the island of Alcaire, in High Rock. Amidst rumors of a warlord destined to rule Tamriel, he rose to prominence in the service of King Cuhlecain and fought to unify Cyrodiil. Few knew of the existence of his secret ally, the Underking, Ysmir Kingmaker. The Underking provided assistance to the young General, but the manner of his assistance is vaguely described in the book. There are references to a storm that talks all night with Hjalti and deflected arrows while Hjalti shouted down the wall of Old Hrol'dan, where Hjalti was given the name Talos, meaning Stormcrown, by his men. This storm is most likely the Underking, as his body had recently been blasted to ash by the Greybeards, and he is referred to as 'Ysmir, the Grey Wind' and 'the Storm of Kyne'. After the province was conquered, Hjalti murdered Cuhlecain and his contingent in the Imperial City and blamed the deaths and ensuing blaze on assassins from High Rock. The Underking continued to aid in his conquests, and Hjalti took the name Tiber Septim, ascending to the Ruby Throne
You can pick your choice what you want to believe about Tiber Septim. I personally chose to go with the version that is shown in the games.