User selecting multiple categories

Hey!

I’m rather new to wordpress theme development, and I’m currently making a site were I want the user to be able to filter posts by selecting one or multiple categories, but I can’t find a solution which works.
To list the different categories I use:
<?php wp_list_categories('child_of=9&title_li=&hide_empty=0&orderby=term_group'); ?>

Example of the list of categories and the output:

Food

• Hot food
• Desserts
• Fruit

Countries

• Germany
• Sweden
• Japan

Month

• January
• November
• December

So the user can select ex. Hot food, Sweden, and January, and only see the posts posted under those categories.

The link would look something like this: http://www.example.com/category/hotfood/sweden/january/

This is the code for the page. (category.php)

<?php get_header(); ?> 

	<?php
 	if (is_page('newsarchive')):
		$temp = $wp_query;
		$wp_query= null;
		$wp_query = new WP_Query();
		$wp_query->query('showposts=8'.'&paged='.$paged);
	endif;
    ?>

  <div class="container" id="nyheter">
  	<div class="sixteen columns">

    <h2 class="sectionTitle" id="nyheterTitle"> Nyheter </h2>

    	<div class="sixteen columns flexslider" id="slider">
                    <ul class="slides">
                        <li>
							<img src="slide2.jpg" />
                        </li>

                        <li>
                        	<img src="slide2.jpg" />
                        </li>

                        <li>
                        	<img src="slide3.jpg" />
                        </li>

                    </ul>
            </div> <!-- end sixteen columns flexslider -->
            <div class="filterGrupperContainter">
              <h3 class="filterNyheterType two columns" id="grupperFilter"> Gruppe</h3>
              <h3 class="filterNyheterCurrent two columns" id="grupperCurrent"> <?php single_cat_title(); ?> </h3>

              <ul id="filterGrupper" class="sixteen columns">
                    <?php
						  wp_list_categories('exclude=11,9,5&title_li=&hide_empty=0');
					      wp_list_categories('child_of=5&title_li=&hide_empty=0&orderby=term_group');
				    ?>
              </ul> <!-- end filterGrupper -->
     	    </div> <!-- end filterGrupperContainer -->

           <div id="filterSidebar" class="fr">
       	  	<div class="filterContainter">
              <h3 class="filterNyheterType two columns omraade" id="grupperFilter"> Område</h3>
              <h3 class="filterNyheterCurrent two columns omraadeCurrent" id="grupperCurrent"> Snertingdalen </h3>

              <ul class="sidebarFilterLinks">
                    <?php wp_list_categories('child_of=9&title_li=&hide_empty=0&orderby=term_group'); ?>
              </ul> <!-- end sidebarFilterLinks -->
     	    </div> <!-- end filterGrupperContainer -->

            <div class="filterContainter">
              <h3 class="filterNyheterType two columns omraade" id="grupperFilter"> Måned</h3>
              <h3 class="filterNyheterCurrent two columns omraadeCurrent" id="grupperCurrent"> September </h3>

              <ul class="sidebarFilterLinks">
              	<?php wp_get_archives( array( 'type' => 'monthly', 'limit' => 12 ) ); ?>
              </ul> <!-- end sidebarFilterLinks -->
     	    </div> <!-- end filterGrupperContainer -->

            <div class="filterContainter">
              <h3 class="filterNyheterType two columns omraade" id="grupperFilter"> År</h3>
              <h3 class="filterNyheterCurrent two columns omraadeCurrent" id="grupperCurrent"> Alle </h3>

              <ul class="sidebarFilterLinks">
			  	<?php wp_get_archives( array( 'type' => 'yearly', 'limit' => 5 ) ); ?>
              </ul> <!-- end sidebarFilterLinks -->
     	    </div> <!-- end filterGrupperContainer -->

           </div> <!-- end filterSidebar -->

          <div class="thirteen columns omega" id="nyheterContainer">

      		<?php if( have_posts() ) : while (have_posts() ) : the_post(); ?>         

            <div class="six columns nyhet" id="firstArticleNyheter">
            	<figure>
                	<a href="<?php the_permalink() ?>">
						<?php if ( function_exists('has_post_thumbnail') && has_post_thumbnail($post->ID) ) {

                       		 $thumbnail = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), full );
                       		 the_post_thumbnail( 'archiveThumb' );
                        ?>

                                <?php }else{ ?>	

                                <img src="images/defaultBilde940px.jpg" height="255px" width="340px" />
                        <?php } ?>
                    </a>
                </figure>
                <span class="date"> <?php the_time('F jS, Y') ?> </span>
                <h3 class="newsTitle"> <?php the_title(); ?> </h3>
                <p class="newsText"> 

                    <?php the_excerpt(); ?>

                </p>

                <div class="newsLink">
                    <a href="<?php the_permalink() ?>" title="Les Mer" class="lesMerLink"> Les Mer </a>
                </div>

            </div> <!-- end six column -->

            <?php endwhile; else: ?>

                	<p> Beklager, det ser ut som om det har skjedd en feil. </p>

			<?php endif; ?>

            <div id="pages" class="thirteen columns">
   				<?php
					global $wp_query;
					$total_pages = $wp_query->max_num_pages;
						if ($total_pages > 1){
						  $current_page = max(1, get_query_var('paged'));
				 		  echo '<div class="page_nav">';
				 		  echo paginate_links(array(
						  'base' => get_pagenum_link(1) . '%_%',
						  'format' => '/page/%#%',
						  'current' => $current_page,
						  'total' => $total_pages,
						  'prev_text' => 'Prev',
						  'next_text' => 'Next'
						));
					  echo '</div>';
					}

           		 ?>

            </div>

          </div> <!-- end nyheterContainer -->
       </div> <!-- end sixteen columns -->
   </div> <!-- end nyheter -->

 <?php get_footer (); ?>

Hope you’ll have a great new year!