Custom sidebar on category pages

one way to accomplish your goal would be to create a child theme (https://codex.wordpress.org/Child_Themes). You create a folder inside /wp-content/themes/, then add a style.css file inside it with a few comment lines at the top to tell WordPress this is a child theme of whatever your current theme is.

Sample style.css file: all you need to do is change parent-theme-folder-name to whatever your original theme folder is. So for example if you are currently using Twenty Sixteen, change parent-theme-folder-name below to twenty-sixteen.

/*
Theme Name: Custom category sidebars
Version: 1.0
Template: parent-theme-folder-name
*/

Next, to create your new sidebars, create a blank functions.php inside your child theme folder, and add:

<?php
/**
 * Add custom sidebars for categories
 */

// Agenda Category Sidebar
register_sidebar(array(
    'name'          => 'Agenda sidebar',
    'id'            => 'agenda-sidebar',
    'description'   => 'Agenda category sidebar'
));

// Articulos Category Sidebar
register_sidebar(array(
    'name'          => 'Articulos sidebar',
    'id'            => 'articulos-sidebar',
    'description'   => 'Articulos category sidebar'
));

Finally, create a blank category-agenda.php file and a blank category-articulos.php file. Copy and paste your original theme’s category.php code right into those files. Then, look for the sidebar portion, and change the part where it calls the sidebar:

Again in this example just change sidebar-slug to your new slug: in category-agenda.php change it to agenda-sidebar, and in category-articulos.php change it to articulos-sidebar.

Once all these files are created and uploaded, activate your child theme and set up the widgets in the sidebars however you like.