widget as link to php file

Hi, all.

I need to make widget (which is part of plugin) on frontend on right side of site as reference link to page of php/js/css-code.
I know how to make widget with reference on right side of site, I know how to make my php part itself. Question is how better organize my file as I want it
to be php/js/css-file and I want it to be located inside of my plugin dirs. I mean something like wp-login.php, file but not in root directory.
Also please explaine how to make my .php file to use all wordpress layout and style as my page must look like wordpress page.

Please, give reference to example or explanation of how to make it, as wordpress is rather new for me…