Not so obvious: Peter established the first Governorship in Siberia in 1708, the first official Russian government presence in Siberia, or "the East".
The problem is that Russia's move to the East, into Siberia took place largely in the 16th and 17th centuries under a bunch of Czarist mediocrities who were mostly concerned with more domestic problems like the Time of Troubles and the Old Believers religious schism. The physical 'conquest' of Siberia was actually undertaken mostly by private means: the Stroganov family, Yermak and his adventurers, and cossacks. Neither the government nor any Czar had much to do with it. Peter's move in 1708 was simply acknowledging moves that had already taken place long before.
IF there was a Czar that started Russian presence in the east it might be Ivan IV Groznyi (the "Mighty" or "Terrifying", not, strictly speaking, "Terrible") who conquered Novgorod, and therefore also 'picked up' all the Novgorodian territories to the east to the Ural Mountains. Mind you, that territory was a little like picking up French Canada - mostly filled with natives and fur trappers, with very little "on the ground" presence from Novgorod, or Moscow, until much later.