What was the main goal of the Spanish conquistadors in their expeditions to the Americas?

When talking about the goals of the Spanish conquistadors, it is also worth noting that their expeditions in the Americas were driven by competition with other European countries, particularly Portugal.
Remember that Spain and Portugal were long-established rivals and that the New World offered plenty of opportunities for wealth because of its natural resources and a native population that could be used for cheap labor. Thanks to Christopher Columbus, the Spanish were the first to gain a foothold in the New World and profit from all of its treasures. Seeing the Spanish benefit financially brought this competition to another level. Portugal wanted a share in the New World bounty.
So, while the desire for gold and glory was a strong motivator, this competition between various countries for supremacy also played a pivotal role in the activities of the conquistadors.
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The Spanish conquistadors were primarily motivated by greed. For many years the European imagination had been captivated by tales of the mysterious land of Eldorado, where gold was in plentiful supply, just waiting for someone bold and adventurous enough to come and take it. The prospect of untold riches was too tempting for many Spaniards to resist, and they headed off to the New World in search of their fortunes.
At the same time, Spanish monarchs sponsored colonial expansion as it enhanced the crown's prestige. For centuries, Spain had been one of Europe's weaker powers, and so territorial expansion in the Americas presented the kingdom with a chance to achieve greatness as well as enormous wealth.
Spanish monarchs also saw expeditions to the New World as a chance to convert heathens to Catholicism. With Europe riven by inter-confessional conflict between Protestants and Catholics, the time seemed ripe for spreading what most Spaniards regarded as the one true religion as far and wide as possible.

The Spanish conquistadors had a few goals that motivated their conquest of the Americas. These goals are often referred to as the "three Gs": gold, God, and glory.
Gold is considered one of the major motivators of Spanish conquistadors. Gold was obviously a valuable resource in the Age of Exploration, as it is now. Spain hoped to gain wealth in the Americas through the discovery of gold deposits and the taking of that gold back to Spain. Thus, we can say that gold/wealth was a motivator of the conquistadors.
God is also considered a major motivator of the Spanish conquistadors. The Spanish believed that the Native Americans they encountered were godless and thus uncivilized. The Spanish hoped to convert the natives to Christianity. The Spanish would eventually establish missions in the Americas to forcibly spread Christianity in the New World.
Glory would be another major motivator of the Spanish conquistadors. Conquistadors hoped to conquer the natives and become known for their conquests, all while gaining land for their homeland. Spanish conquistadors could often become famous in Spain for achieving these conquests.
When examining the motivations of the Spanish conquistadors, they are not very different from what motivates people in the modern day. Many people are motivated to do certain jobs or take certain actions in order to gain wealth, gain fame, or serve religious purposes. However, most modern people (hopefully) do not participate in such harmful and genocidal systems as the conquistadors did.