Well I have to prosecute Stalin in a "court case"/debate for starting the Cold War and I don't quite know how to go about crafting my arugument. I need a solid argument with real evidence.
I think that your overall argument has to be that Stalin caused the war by trying to spread communism in and around Eastern Europe. You can say that he took aggressive actions and that he failed to keep his promises.
At Yalta, Stalin agreed to allow free elections in Eastern Europe. However, he soon went back on this promise. Instead, he did things like forcing Poland to have a communist government without any chance for elections.
Beyond that, Stalin started to try to undermine countries that bordered the Soviet bloc. For example, Stalin started sending aid to people in the region (now a country) of Azerbaijan. Those people were trying to break away from Iran.
By breaking his promises and by trying to undermine countries that bordered Eastern Europe, Stalin made the West need to worry about communist expansion. This, you can argue, is what caused the Cold War.