Creating first module, and How to alter text field on new content creation in Drupal 7?
A couple of pointers:


Use only lower case for module names.
$node is not available in hook_field_presave(). $entity should contain necessary info.
hook_nodeapi() is not available in Drupal 7.
It's possible you can make these alterations in your theme's template.php file as well.


You can use the devel module to help with debugging, e.g. dpm($entity, '$entity contains').

Also see Module developer's guide and Drupal 7 API reference for more info.