paging and category

Hello,
Lately I have dabbled in wordpress and my questions may look funny but I cannot find solutions for them now.

Firstly, where can I activate the paging and define the publications’s number on a single page?
Secondly, the category’s menu’s html structure is:

<ul>
   <li>
      <a>cat_name</a>
   </li>
</ul>

what and where should I change to become like this:

<ul>
   <li>
      <a>
         <span>
            cat_name
         </span>
         <span>
            category_description
         </span>
      </a>
   </li>
</ul>

Thanks in advance and I am sorry if the topic is not placed correctly.