Well, the Red Army had millions of soldiers in it, while the White Army never even had 1 million. This huge imbalance played a huge role in the defeat of the Tsarist forces. Also, the communists controlled the major industrial areas like Moscow, so they could produce more.
As for stopping communism, it was pretty simple. Mikhail Gorbachev, the last leader of the USSR, was the only Soviet leader who I actually liked. He let the republics vote whether they wanted to remain communist or gain independence. All the republics chose to be free, and the USSR collapsed.
Also, fun fact: Russia wasn't the last republic to leave the USSR, it was actually Kazakhstan. Kazakhstan was the entire USSR for a few days actually