Displaying User info in Sidebar Widget

I am looking to display user information in a sidebar widget when the user is logged in. Users do not have access to the wp-admin part of the site, so I want a widget in the sidebar to say something along the lines of “hello, username!”, then place additional links below. How can I do this? If I am using CSS or code that should go in a .php file, where would I add it? How can I get this info to display in a sidebar widget?

Thanks for any feedback anyone might have!