Thanks in advance.
To ease your JS adventure, check out JQuery.com which makes it easier to code JS without worrying so much about browser quirks.
For easy server-side login, I recommend PHP since there are literally millions of tutorials and free code snippets out there, hosting is cheap, and an enormous community of PHP developers online to help you. To ease your PHP adventure, start with a framework such as CodeIgniter.
In order to upload files, you'll need a server-sided script, possibly done in PHP.
As for communicating with the server, it can be somewhat daunting for a beginner, but it's largely done via
xmlhttprequest (otherwise known as AJAX).
Here's a tutorial on how to use it by W3CSchools, but there are plenty more on the net: http://www.w3schools.com/ajax/default.asp