To identify landmark papers, you can also consult the bibliography of your textbook if you have already taken classes that cover (even briefly) your subfield of interest. If your textbook is one that is used by a lot of university professors, then it probably covers landmark papers well. You can often identify the most important papers in a textbook because they'll possibly:
- be referred to multiple times and in multiple chapters
- appear near the beginning of a chapter when a topic is first introduced
- explicitly be acknowledged in the text by the author of the textbook as a seminal paper
A disadvantage to this approach is that, depending on when your version of the textbook was last updated, it may miss important developments in recent yeras.
Identifying highly-regarded authors can be harder, but you can try looking up authors (especially last-named ones, who are often supervisors/professors) of important papers on, for example, Google Scholar, to see what else they have produced and whether they have a lot of highly cited papers.