How do i know if someone logged into my fb from another computer and will i be able to know who did it\/

what the hell ?? what are you talkin about? FB is rubbish , get off it and get a life as it is a waste of time. Get on the phone and tell em if ya got somtin ta say not a FB msg...

Someone in all likelihood has hijacked your FB account. Create a new password.

Try rebooting computer or using a program like Ccleaner to remove cookies.
It's free and safe to use, use the cleaner and don't be afraid to delete all the junk it finds, then run registry scan for issues.
https://www.piriform.com/ccleaner/download/standard

Log out of their account by clicking the little gear icon in the upper-right then clicking "Log Out."

Click the little gear icon in the upper-right then click "Log Out."

Just log out of the other account by clicking the little gear icon in the upper-right then clicking "Log Out."

You need to sign out of their account on your computer. Just click the small gear at the top right of the page and then click Log Out in the dropdown menu.

Once you have logged out of the unwanted account you can then sign in with your own account. If you're on a public computer don't forget to log out of your account when your done or the next person will be signed into your account if they visit facebook.

Hope that helps!

if you're able to log in from one device, either your phone or through your computer you should be able to get it unlocked yourself or do you have problems on both devices? You can also try the recovery option: FB

Check the address bar to make sure you're not being phished (directed to a fake page to steal your login credentials).

Or

Reset your password, and login again.
http://www.facebook.com/reset.php