The wiki page does an excellent job of outlining everything including mentioning that the actual start of the Cold War is up for debate and some saying it truly started as far back as WWI: http://en.wikipedia.org/wiki/Cold_War

But in short form, a few events in late 1946 helped solidify the western world against the Soviets, then in 1947 the Soviets led the formation of the Cominform which was basically a loose, unofficial alliance of all communist states. From the wiki: "The intended purpose of Cominform was to coordinate actions between Communist parties under Soviet direction."

It was also the year that the U.S. adopted its containment policy against the spread of communism. So its not so much what happened in 1946 as what happened in 1947, as to why that year is usually considered the start, even though the tensions had been in place for years prior.

Also, basically things escalated to that point after the war (and not prior) as both the US and the Soviets were vying to rebuild a devastated Europe into being either pro-communist or pro-capatilist.

The concept of a "start" to the Cold War is nebulous at best. The reason for this is, of course, that there was no official "declaration" of war which would legally constitute the start. Therefore a determination of the war's "start" in terms of a specific date is pretty arbitrary. One can trace the straining of relations and hostility between the USSR and the west back to the October Revolution.

With that being said, Truman's famous speech on March 12, 1947 is commonly cited as the "start" given that it verbalized a consistent American policy towards the USSR which formed the basis for the relations between the two countries going forward.

Regarding what led up to that speech, please see my comment here: http://www.reddit.com/r/AskHistorians/comments/1xjbsw/how_can_historians_claim_stalin_did_not_cause_the/cfcq3mu?context=3

I strongly believe that between the end of the war and this speech, conflicts of interest quickly degraded the relations between the two countries, though I think the US was more aggressive in this practice, and that this speech by Truman was intentionally meant to be a very aggressive speech, essentially with the intent that it ultimately had.