Short answers, pulled from the story.
Mikhail Gorbachev was elected General Secretary by the Politburo on the 11th of March 1985. He became the youngest member of the ruling body at age 54 following the death of his predecessor Konstantin Chernenko.
Lithuania became the first Soviet Republic to declare full independence from the Soviet Union on the 11th of March 1990 through the Act of the Re-Establishment of the State of Lithuania. Estonia had previously declared state sovereignty inside the Union on the 16th of November 1988 but Lithuania was the first to restore full independence.
The Soviet Union was formally dissolved as a sovereign state subject to international law on the 26th of December 1991 by Declaration No. 142-N of the Supreme Soviet of the Soviet Union. This followed Mikhail Gorbachev's resignation on the 25th of December 1991 and the subsequent vote by the Soviet parliament to dissolve the union.
Popular Front organizations such as Sąjūdis in Lithuania, the Popular Front of Latvia, and the Popular Front of Estonia were established between May and June 1988 to challenge Soviet rule. These groups successfully pressured liberal Communist leaders into legalizing national flags and adopting language laws that asserted local sovereignty over central authority.
Communist hardliners attempted to overthrow Mikhail Gorbachev during the failed 1991 August coup which resulted in the central government losing influence. The turmoil allowed Boris Yeltsin to emerge as a dominant figure who later became the first president of the Russian Federation after Gorbachev resigned on the 25th of December 1991.