How to add a menu on a wordpress theme??

I want to add a menu horizontally on the orginal theme like a lot of websites have them. How do I do that??

 

add <li><a href="url of catagory or page </a><li> add this code in the header file
are you have some knowledge of php?

 

Let me guess, you use wordpress template
where it means you only to create a new page,
later you'll have the navbar based on what the template has provided.

Alternatively,
you can create your own navbar with some CSS and HTML trick.