Similar questions have been asked before (This is one such example). But I will go into some of the broad details of the matter.

For starters we must consider the context of the Molotov Ribbentrop Pact. It's 1939 and Hitler had already annex Austria and gobbled up Czechoslovakia in the years prior. During those years one of the diplomatic goals of the USSR was to contain Germany, whom they (correctly) viewed as the largest threat in Europe. To these ends there were several attempts at alliances with the western powers (particularly France and England) in order to counter Germany. However these negotiations went nowhere. While there was officially an alliance with France and the USSR signed in 1935, the conditions and stipulations made it so weak as to be practically useless. A stronger alliance was attempted but would regularly reach an impasse, with the primary sticking point being Poland. Namely that the UK and France believed that, should the Soviets move their armies through Poland, they would be unlikely to leave.

All this meant that, for the Soviet Union/Stalin, the past three years had been spent with the Western powers refusing to form a coalition against Hitler and essentially giving him a free pass to expand and conquer. Now the USSR had a much more powerful Germany right on its doorstep and wishy-washy Western powers nominally opposing it. It was obvious that Germany was planning to attack Poland next, and (presumably) the Soviet Union eventually. Thus the Molotov Ribbentrop Pact was made, which was functionally beneficial to both the Nazis and the Soviets. To clarify a common misconception: the Molotov Ribbentrop Pact was NOT an alliance. It was a non-aggression pact which agreed to recognize certain territories and "spheres of influence" and agree to not go to war for a set amount of years.

For the Nazis it meant that they would not have to fight a two front war against the allies and USSR, which they were certainly not prepared for. For the Soviets it meant that they would have time to prepare and modernize in anticipation of an eventual Nazi attack. It also gave them a free hand to reclaim territory they had held prior to the Russian Civil War (Eastern Poland, Bessarabia, the Baltic States, and Karelia). Simply put: it was a pact of convenience/realpolitik. The Nazis were certainly not ideal to work with diplomatically for the Soviets but the Western allies had proven themselves too weak and inflexible to the eyes of Stalin. Of course, Germany would surprise Stalin by breaking the non-aggression pact pre-maturely and attacking the USSR even while still at war with the UK. This was similarly done because the Germans were aware that the USSR was modernizing and improving its military.

So in short: the pact was made because it was mutually beneficial to both countries, ideology aside, and alternative diplomatic plans for the Soviets had proved unfruitful.

As for the question: Was this Stalin's first step to conquering Europe, that is frankly absurd. Obviously we cannot read Stalin's mind or know 100% what he was thinking or planning. But his policy of "Socialism in One Country" as well as his handling of the postwar diplomacy certainly run counter to the narrative that he was trying to take over the world. He certainly had territorial ambitions (see before mentioned reclaimed lands) but it would be safe to say that he had no serious intentions of conquering all of Europe, he didn't even bother taking Finland for instance, even though after WW2 he very well could have without too much difficulty.