The easiest way:
Go to wp-admin at Posts > Categories > select a category > Edit > in the description area, write your desired text.
Now open the page corespondent to your categories (it should be something like category.php or archive.php, from your theme’s folder) and add this php code: < ?php echo category_description(); ? >. (remove the blank spaces).
You can even bold your description text by adding “b” or “strong” tags. However, you can’t style your text with css code like style=”color:red;” etc.
If you know other ways, please leave in the comments.