Problems login into websites

I can't login into websites on my iMac
It started with some sites, but today i can't login into every website i tried.
When I put in my information, it takes me right back to the login page. no error, no message, just right back to nothing.
I can't even logging into gmail anymore !!
I know the information is correct, because i tried that on my iPhone and PC.

I've tried several things for now, such as:
- emptying Cookies, than logging off and restarting the computer.
- using different internet browsers such as Mozilla Firefox and Explorer.
- shutting the firewall.

I'm new with a Mac and i'm not good in Compu-talk...
So please if you answer tell m as simple as possible