You need to use the XSSFWorkbook object to create a new workbook. (org.apache.poi.xssf.usermodel)
workbook = new XSSFWorkbook(new FileInputStream(FILENAME));
Then you can assign Sheets accordingly, using getSheetAt() method.
An FAQ, please refer one of the several blogs on the topic.
"Using Apache POI to Read Excel"
"Introduction to the Apache POI Library"
"Working with Microsoft Excel in Java"