I would like to add a piece of code over/before the posts on tag and categorie pages.

it is to display a categories or a tags geolocations of postings for the plugin geo-mashup.

any idea to get that piece of code over the first posting of that tag/cat page?

You could put, into index.php, right after

PHP Code:

<?php get_header(); ?>

this:

PHP Code:

<?php if ( is_tag() OR is_category() ) { ?>
YOUR CODE
<?php } ?>

To have it only on the first page of tag/cateory lists

PHP Code:

<?php if ( (is_tag() OR is_category()) AND !is_paged() ) { ?>
YOUR CODE
<?php } ?>