Short answers, pulled from the story.
The Commonwealth of Independent States formed on the 8th of December 1991 when three leaders met in a forest reserve near Brest in Belarus. Boris Yeltsin, Leonid Kravchuk, and Stanislav Shushkevich signed a document at the Belovezhskaya Pushcha Natural Reserve to declare the Soviet Union ceased to exist.
Ivan Korotchenya became the first Executive Secretary on the 26th of December 1991 and later served as General Secretary from the 14th of May 1993 until the 29th of April 1998. Sergei Lebedev has held the position since the 5th of October 2007 following terms by Vladimir Rushailo and Yury Yarov.
Eight of the nine current member states participate in the CIS Free Trade Area while Turkmenistan holds associate member status due to its declared international neutrality. Georgia withdrew its membership in 2008 and officially left in August 2009 after a war with Russia.
Ukraine formally ended its participation in CIS statutory bodies on the 19th of May 2018 after stopping active participation in 2014. The country never ratified the Charter making it a founding state but not a full member despite the secretariat receiving no formal notice regarding withdrawal from the Creation Agreement as of the 1st of June 2018.
Six post-Soviet states signed the Collective Security Treaty in May 1992 which took effect in 1994 before evolving into the Collective Security Treaty Organization in 2002. CSTO members agreed to create a peacekeeping force in 2007 and include Armenia, Belarus, Kazakhstan, Kyrgyzstan, Russia, and Tajikistan.