I'm very, very new to Amazon EC2, SFTP -- having only used FTP clients until now. I'm trying to log into an Amazon EC2 instance and have everything I needed except a pem file with the key pair, which I have now. However I was told to chmod and reset its permissions to 400 in order to log in correctly. The problem is I have no idea how to go about doing this. There is talk of just entering chmod 400 keyfile.pem through the command line, but is that the Windows command line on my desktop? How can I do this? Any help would be much appreciated...
Either log in using an SSH client (like PuTTY).
Using it, you can execute the command you mention (chmod 400 keyfile.pem) on a command-line.
See also SSH to Amazon EC2 instance using PuTTY in Windows.
Or you can use a GUI SFTP client (like WinSCP) to set the permissions.
See https://winscp.net/eng/docs/ui_properties
Make sure only the R checkbox in Owner row is ticked (that's an equivalent of the 400 permissions in an octal format).
(I'm the author of WinSCP)