To do this you will need to be able to edit the index.php file found in your themes folder. You will also need to be able to download the file, edit the file then upload the file, or set the permissions so you can edit it within wordpress. Either way, what you need to do is add one line of code to your index.php file:
Somewhere in the code for the index.php file you will find:
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
Just before that line of code, add:
<?php query_posts('orderby=date&order=ASC'); ?>
More information on how to use the query_posts() function is here:
http://codex.wordpress.org/Template_Tags/query_posts
in case you want to change anything else about how your posts are displayed.
