The headers already sent message is often a result of a php error earlier on in the c5 rendering process, before it gets to the point where the actual page complains about headers.

Look back in your php error log to see if there are any previous error messages.

Also look in the browser developer console, to see what those 'already sent' headers are.

Perhaps you have been working on other code or installed/uninstalled something that is causing such errors before c5 gets as far as your login page.

Where can I find my php-error-log?
What is the 'browser-developer-console'?

You see, I'm just a user!!!

Some Howtos for background

http://www.concrete5.org/documentation/how-tos/designers/absolute-b...
http://www.concrete5.org/documentation/how-tos/editors/getting-help...
http://www.concrete5.org/documentation/how-tos/editors/the-block-do...

The php error log depends on how your web server is set up. If it is not accessible from your host control panel you will need to ask your web space host's support people.

Without developer knowledge it may be easiest for you to attack the problem from the other end. Start making a list of anything and everything you have changed on the site, going back in time from now, even (and especially) if you think it has nothing to do with the login.