Order of pages

Take a look at the sort_column and sort_order parameters in the template tag, wp_list_pages.

Additional Resources:
Template_Hierarchy
Stepping_Into_Templates

I can try, although all the information is described better on the Codex pages that MichaelH linked to. Did you really read it or just had a brief look? There is even a section about your question – 4.2.

Open your sidebar.php or whichever file you want to modify (as a lesson for future: don’t ask layout questions without referring to your theme. Takes out the guessing…).
Find the above mentioned wp_list_pages tag.
As described in the Codex, add the proper parameter.
Save. Upload. Done.

P.S. and there is even a plugin:
http://wordpress.org/support/topic/86044?replies=7