You should take a look into creating custom meta boxes to display the text fields in the admin & then save that data as post meta. There are many tutorials available online for this
Once you have the information, you can put a
get_post_meta call in your theme's sidebar.php file & display the data there directly. If you want it in widget format, you'll have to create a custom widget to display the meta information. Again there are many tutorials available online for this.
When displaying the info you can add a condition like
if(is_single() && 'post' == get_post_type())
echo get_post_meta(get_the_ID(), 'custom-field-key', true);