Why and how did the USSR establish control over the countries of Eastern Europe 1945-1948?

does anyone have a good website where i could find the answer to this? thanks!

Basically it was the meeting at Yalta.

For more detail:
http://en.wikipedia.org/wiki/Yalta_Conference

The Berlin Blockade of 1948 to 1949 became one of the earliest tests of the U.S. Cold War policy of containment of the Soviet Union. The U.S. response to the challenge, history's largest exclusively aerial supply effort in the form of the Berlin Airlift, was initially designed as a temporary measure but became symbolic of American ingenuity and resolve.

The crisis had its origins in World War II–era agreements on the occupation of postwar Germany that placed a jointly occupied Berlin approximately 110 miles within Soviet-occupied territory. Between 1945 and 1948 relations between the international forces stationed in Germany deteriorated, with the growing conflict coming to a head in mid-1948. Protesting the merging of the British, French, and American occupation zones, Soviet officials in Germany walked out of the Allied Control Council, the quadripartite governing body, and the West implemented currency reform in the Western zones. In response, the Soviets announced that they were taking steps to preserve the economic integrity of the Soviet occupation zone and that effective 24 June "technical difficulties" would prevent land access between the Western zones of Germany and Berlin.

Once it became clear that the Soviets had in fact blockaded Berlin, President Harry Truman responded with a firm decision that the United States would maintain its right to be in Berlin even at the risk of war. As a temporary effort to maintain the small Western military garrisons in the city, British and American forces implemented a small-scale airlift. Flying along air corridors established under separate agreements with the Soviets, the airlift was gradually built up to sustain the population of 2.5 million civilians in the Western sectors of the city, an effort code-named Operation Vittles and directed by U.S. Military Governor General Lucius D. Clay.

Mounting an airlift on such a scale proved an enormous logistical challenge, particularly in light of U.S. postwar demobilization. During the airlift over a quarter of a million flights supplied the city with basic needs, including food and coal, at a rate of a plane landing every two to three minutes around the clock. Although Soviet forces harassed some flights, they never did so to the extent of interfering seriously with the airlift. Inclement weather and fatigue ultimately proved more dangerous, and the operation cost thirty-one American, thirty-nine British, and nine German lives. The impasse was finally resolved through informal talks between the American and Soviet representatives to the United Nations, and the blockade was lifted on 12 May 1949. Beginning 1 August 1949, the airlift was gradually phased out over a three-month period.

http://www.answers.com/topic/berlin-blockade

Roosevelt, Churchill, and Stalin virtually took a map of Europe and divided it among the three major powers at the Yalta Conference.

WHY?

[1] In order to grab control over as much as possible of the relatively richer, more advanced (compared to Russia) countries of Europe. Imperialism, in fact. Russia had been devastated by fighting in WW2. Although many of the countries that Russia occupied in 1945 had not been enemies of Russia in WW2, the Russians still dismantled entire factories and ransacked homes (even pulling plumbing fixtures out of walls) to shipped them back to Russia.

[2] In order to establish a perimeter of "buffer states" between Western Europe and Russia itself, so that no invader could attack Russia directly. Stalin was paranoid about that prospect. And, I suppose that was understandable, given the way that Hitler had treated him when they were supposed to be friendly neighbors.

[3] To spread worldwide Communism. No country ever became Communist peacefully, through fair democratic elections. Communism always had to be forced upon a country via "the barrel of a gun". Stalin knew that none of Russia's neighbors would voluntarily accept Communist regimes. The only way to achieve that end was to occupy them, and have the Red Army and the NKVD (which later became the KGB) make sure that Communists won the elections.

HOW?

[1] Quite simply, the Red Army filled a vacuum left by the retreat (and later the destruction) of the German Nazi Regime. All of the European countries and territories occupied by the Red Army in 1945 had formerly been either occupied by or allied with Germany. With the German armies destroyed largely by the Red Army itself, there was usually no other power in the land left to take control.

[2] Poland was an exception. There a large, popular and well organized resistance movement against the Germans, the "Home Army", was still active after the Germans retreated; and there was a Polish government-in-exile (based in London), ready to take power. In order to make sure that the Poles did not remain truly independent, Home Army units were disarmed by the Russians; then the Home Army soldiers were sent to Russian concentration camps (or simply executed by the NKVD). And the leaders of the Polish government-in-exile were lured back to Poland, where they too were arrested and then executed.

[3] There was resistance against Russian control in other occupied countries too. Czechoslovakia, for example, tried to assert its independence through entirely peaceful means. But again, the Red Army and the NKVD went to work, and within a couple of years, every one of the countries occupied by the Russians in 1945 had been transformed into a puppet state, getting its orders from Moscow.

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As other Answers have stated, the territorial division between the Soviet sphere and the spheres of the Western Allies had been pre-ordained at Yalta (over Churchill's strong objections, but with Roosevelt's blessing). At Yalta, Stalin had promised that there would be "free and fair elections" in all of the countries assigned for occupation by the Red Army. And there were, indeed, elections. The only snag was that they were neither free (of Russian interference) nor fair.