Freediving is, if done wrong, dangerous business. Advanced freediving needs to be done in a safe environment among skilled freedivers.
A course in Freediving will not only teach you the basics of safety, but also set you of at a high level in just a few sessions. What can take years to learn by yourself will take a few days with a skilled instructor.
There are many kinds; instructors who themeselves can dive deep coming from a background of competetive freediving, others are more shallow, but hopefully make up for lack of deepdiving skills with good pedagogic skills.
Even though many factors are known by all these instructors - freediving is still in parts a mystery. Your main equipment is your body and mind.
You may chose to take a course guided by strict levels and manuals or a course more personalized for you. Some teachers see freediving in a more technical way, others has a more holistic approach.