Showing membername in header

I’m trying to incorporate an easy log-in/log-out system in the header.php template on my custom theme. I simply want the theme to display the username if someone is logged in with the option to log out and if they are NOT logged in show a username/password form to quickly log in from any page within wordpress. I tried hacking a bit of the php/html from the default wordpress theme comment.php to do this but to no avail. I just can’t seem to get wordpress to identify if someone is logged in. I understand the simplicity of the user registration system, I’m just wanting an easy way for visitors to log in/log out.

How would I do this? Thanks ??