Best Many things led to Revolution (and one interesting question is "Had Tsar Alexander II not been assassinated, would Russia be a constitutional monarchy even to this daybut for another time), but the three main things were World War I going badly for Russia, Rasputin's perceived influence on Tsarina Alexandra, and the fact that Tsar Nicholas II was pretty much whipped by the Tsarina and would do anything she asked just to keep her happy. All these were very much interrelated.
As World War I went horribly for Russia (along with thousands of mutanies along the front), Nicholas decided to go to the front...which of course made matters worse. Alexandra was pretty much left in charge, and since she was German by birth, there was already much suspicion. To further alienate the people, her inner circle was Rasputin and Anna Virubova, the former causing much concern among everyone. The Tsarina overlooked Rasputin's debaucheries as he seemed to be the only one who could heal the heamophiliac Tsarovich Alexi. Rasputin had a nack for knowing what the Tsarina wanted to hear, and would make suggestions on policy based on this sense. She would then send these letters to the front and Nicholas would agree to whatever she asked. With her being German and Rasputin's reputation, the people finally had enough. Nicholas abdicated the throne to his brother, and the revolution occured shortly thereafter.
These are the immediate causes that led to the first revolution (of course, revolutionaries have abounded since Alexander II freed the serfs in the 1860's attempting to kill him seven times).
Lenin came into the picture thanks to the Germans, who looked for a way to remove Russia from the war to free up troops for the bogged down western front. Lenin was leading from exile (although politically brave, he was actually a personal coward). The Germans secretly furnished a train to get him into Russia to stir things up. He eventually led Russia into civil war following the Bolshevik revolution.
Edit: Cruxthe white army was the army that followed Trotsky in the Russian civil warnot the actual Russian army in World War I.
