Add search box & category selection in menu

Dear all,

I am completely new to WordPress.
I installed the Cubic theme and can create posts and this all works just fine, but I do not know how to add search capability in the menu (search on tags). I also want to add post selection by category, such as what is done on this website:
https://csoneats.com/2016/03/25/taipei-eats-%e6%a2%81%e8%a8%98%e5%98%89%e7%be%a9%e9%9b%9e%e8%82%89%e9%a3%af/

Should I add a plugin ? If so which one ?

Thank you for your support.

Sebastien