How were the Communists able to take over Russia?
Russia was in very bad shape in the late 1910s. The economy was stagnant. Corruption in the government was rampant. There were severe food shortages, which exacerbated civil unrest around the country. The Czar was extremely unpopular.
But the main triggering event for the revolution was World War I; for many Russians this was simply the last straw. The war killed thousands of Russians and crippled the already weak Russian economy; people finally just got fed up. It began with labor strikes, which escalated into riots, and finally it became a full-blown civil war.
There were really two revolutions; the February Revolution (actually in March because Russia was still on the Julian Calendar at the time) created a provisional government, but it was unable to stabilize Russia or end the food shortages, so it in turn was overthrown about six months later by the Bolsheviks, led by none other than Lenin, who promised an end to the turmoil and "peace, land, and bread" for everyone. Once the Bolsheviks consolidated power, Lenin became the first head of the Soviet Union.