Maria:

This is the link to the Ledger manual: Ledger 9.0 manual

If you look at page 138 (chapter 4, page 30), you will see detailed notes. But you should also look at the sample data that comes with Ledger. It will have both pre-defined templates and sample import files. Try them out.

If you still have questions, post further but tell us your version of Ledger and specifics about the file you are planning to import (source?, format?).