There are two good working WordPress plugins available that allows you to display recent or random posts from a set of tags or categories in your WordPress site’s widget areas. All you need to do is Install-activate them and you display posts by categories and tags from Appearance -> Widgets page.
For Displaying Posts By Tags
Install and activate Posts By Tag and use its widget (as many times you like) from Dashboard ->Appearance ->Widgets page.
For Displaying Posts By Categories
Install and activate Category Posts Widget and use its widget (as many times you like) from Dashboard ->Appearance -> Widgets page
![Display Posts by Category in WordPress [widget]](https://i0.wp.com/s.wordpress.org/extend/plugins/category-posts/screenshot-1.png?resize=261%2C507)