Stalin was not his original name, but he changed it to Stalin - it means "man of steel". Lenin didn't like him much, and certainly didn't want to to rule the country; he said as much in his letter - he called him "too rude, and too arrogant" to be leader.
He became General Secretary, and in that position started his "ride to the top" - he promoted people loyal to him, rather than those competent. This meant that he soon had a whole section of the Party under him, and they would support him rather than anyone else. When Lenin died in 1925, he used these people to get to the top. He also played each side (pro vs. anti) against each other until they killed themselves, and then stepped into the gap and took over.
He WAS a bastard; he knowingly used slave labour from the camps, and had his enemies eliminated. He became paranoid that they were all out to get him; this started the purges. In the purges, 100 000 people were taken away, many of them killed. The Party staged "Show Trials", in which the accused, a purge victim, was charged with numerous crimes, most of which were never committed. "Enemy of the People" came into its own, where people did not need trial because they were said to have committed crimes against the state. The Show Trials were effective, however, because the victims were physically and mentally threatened by interrogators until they confessed. In this way Stalin was able to send many able, innocent people to either imprisonment, labour camps, or their deaths, "legally".
The Stalin Constitution (1936) was designed to make the Party look like it was running a free democratic government. The parliament was replaced with the Supreme Soviet, of which there were two chambers. The first was the Soviet of the Union, and it had 1 deputy for 300 000 voters. The second was the Soviet of Nationalities, and it had one deputy per republic. There were elections for both every four years, but the candidates had to be approved by the party. In this way Stalin protected himself from those that could hurt him; he set up the elections, but the candidates all had to be approved. It wasn't really a democratic system at all.
The people were promised freedom from arrest without trial, and that paper and printing materials would be made available to workers to freely publish their ideas. This was also meant to appear to be democratic, however, neither of these promises were kept and the people got nowhere.