category list on main index

A link to your site would be helpful and also what theme you are using.

BUT, if I had to guess I would say somewhere in header.php.

wordpress default template,

its on localhost at the moment, is there a particular area to place the catetory if its on the main page index

It all rather depends on how you structure your code and setup the css…

The most likely place will be in the header.php file of the active theme, however I have one site where the navigation is physically in the footer.php file, but then uses css to place it just below the masthead and above the content when viewed in a css-aware browser.

What you will have to do, assuming that you want the navigation to run horizontally, is to set the css styling such that list items in the navigation display inline, or are floated.

Hope that helps.

This should help a lot:
https://codex.www.ads-software.com/Creating_Horizontal_Menus

In the default theme, if you want the list *in* the blue box, right under the title and description, go to the Theme Editor, click on Header (right side list of files) then scroll down almost to the end. you should be able to plunk it in right after the blog description div.