Ellis Island is useful for diaspora, descendants of those who have immigrated to the United States. Ellis Island is a harbour in New York close to the Statue of Liberty. From 1892 until 1954 this Island was used as a terminal for those who wanted to settle in the United States. When new immigrants arrived they were subjected to a medical examination. Only those who were qualified were allowed to enter, others were send back. On the website of Ellis Island more information can be found about the Island that played such a crucial role in the history of many families.
Of all those families that entered records were kept. These records have been scanned in an can be accessed through the internet. Many errors have been made in the days when people entered the US as immigrants. Names were often not spelled correctly. So under no circumstance copy and paste the information you find on the passenger lists. Also note that the historical context was much different then today. The troubled relationship between Croats and Serbs wasn't actual as today. Croatia was part of the Austro-Hungarian Empire, and as such many Serbs were listed as being from e.g. Croatia. It is important to keep in mind the distinction between nationality (statehood wise) and ethnic background.
In order for you to do research in Serbia about your ancestry you must first have depleted the researches in the US. Double check the data that you have with personal items, family pictures, documents and other items in your family's possession. Only after determining the correct spelling and date of birth of your ancestor we advice you to start writing the archives.
http://www.ellisisland.com/ Main Website
http://www.ellisisland.org/ Island Records (free registration)