how can i do a header like this for post

Hi there,

create a new Header Element for your Posts:

https://docs.generatepress.com/article/header-element-overview/

You can use the Template tags to dynamically display the various content.

how to do that exactly , can some1 help me to set it up

Follow that document on how to set-up the Header Element.
Then add this to the Header Element content:

{{post_terms.category}}
<h1>{{post_title}}</h1>
{{post_author}} • {{post_date}}

It will automatically display the Category followed by the Title, Post Author and Post Date.

On the Display Rules tab of the Header Element set the Location to Posts > All Posts.