Adding Categories

I am very, very new to this and I’m totally lost.

My site is here:

https://reddingcomputer.wordpress.com/

You can see that I’m trying to add links to the header that lead to posts that correspond to the area the reader clicked. If you click on Linux News, you should get a page with articles about Linux.

I tried adding categories but the categories don’t show up on the page.

I don’t want just a page of running posts going on forever.

Is there some way to add posts according to categories so readers can find what they want to read?