You need to give tar the correct path to your file - I've no idea where you file is right now. Also be careful where you extract to, as tar might overwrite existing files which may or may not be what you intend.

Assuming you're in the same directory as your archive file (eg. myfile.tar), if it's not compressed you should be able to list the contents with

Replace "myfile.tar" with the path to your file.

Does this work?