After the first exciting experiences you are finally set to get your first certifications !!! There are a few more steps to follow and you may soon become member of an ever-increasing international community of scuba divers.
Based on age and time available there are options in what you may decide to get while in Sardinia. In any case the progress is in steps and both ESA/CMAS and PADI both give you 2 options :

1) Certification to 12m - recommended for the youngest and for limited time available - a course that takes 1 or 2 afternoons and 2 dives to include:
a manual/kit (digital), simple and fun theory personalzed with the instructor 1 or 2  dives in shallow waters to learn the necessary skills for safe diving
Two dives out at sea in close contact with an instructor
--- international certification to be able to dive anywhere down to 12m together with a dive professional

2) Certification to 18m. This course can be the continuation of 1) with another 2  days of extra training (in total 4 days to include first experiences) including :
a manual/kit (digital), theory personalized with the instructor and final exam
Five training sessions in shallow waters (2 or 3 beach dives) to learn the necessary skills for safe diving and autonomous diving (can be done in 2 days)
Four dives out at sea in close contact with an instructor (also possible in 2 days)
--- international certification to be able to dive anywhere down to 18m indipendently

ESA training allows you a ISO/EN internationally recognized certification using the latest standards for safety and personalized training.