Stalin started out as a Secretary for the Bolsheviks, he used his office to get incriminating information on all of the Party members and ultimately used this to gain power.
Stalin clearly started out as a lot more than a secretary. For instance he masterminded a massive bank robbery that funded the early Bolsheviks.
I highly recommend this book. It is extremely well researched and fascinating. I had always assumed that Stalin got where he was solely by ruthlessness. But he clearly was an extremely talented and intelligent youth and probably read more books than most of us here, though obviously this all turned in the wrong direction.