I want to shorten order number (increment id) but I don't want to use any modules.
To reduce the number of digits, you'll want to modify the
increment_pad_length in the
eav_entity_type table (for the
order row, of course).
You can change the prefix by modifying the
increment_prefix value in the
eav_entity_store table. (Use the
entity_type_id from the other table to identify which row you'd change.) By default, Magento uses the store's id (
1) as the prefix.
If you're interested in learning more about the increment IDs, check out the
Mage_Eav_Model_Entity_Increment_Abstract classes which are responsible for generating the next available ID based on the database settings.
Edit: You could even create your own increment logic by extending the Abstract class I mentioned above.