Question: how to become the leader of the thieves guild?
Answer: To become the guild master you need to help the guild regain a foot hold in Skyrim. This can be done by completing enough of Delvin's and Vex's radiant quests, and then completing the special quests. These special quests are unlocked after doing 5 quests in the main cities, Whiterun, Windhelm, Solitude and Markarth.