Use the following Subject Headings on the library catalogue to search for resources about this topic
Start with a narrow search (this will depend on your topic) for example :
Explorers, Spanish
Pizarro, Francisco (names of individual conquistadors/explorers)
Incas - History
Aztecs - History
If you are having trouble finding information or want to find more, try a broader search for example:
South America - Discovery and exploration
Explorers
Discovery and exploration
Spain - History
910 Exploration
972.02092 Mexico - History (Aztezs)
985.02092 Peru - History (Incas)