I need that new submissions for an specific content type have a consecutive number as title. Found auto_nodetitle, but I don't know how to make it a consecutive number, they don't give clear instruccions as how to program it. I'm fairly new to Drupal.
If you haven't installed Token module, do so.
From there, you can insert the token [node:nid] into the auto title field. If you don't want to use the nid, you can install the Serial field module, add a field to the content type, and insert that into the title via [node:field_machine_name]. For most purposes, nid should suffice.
You can browse & insert token values via the supplied token browser ui.