Hi Katrin, you r refering to scuba diving right?
You can check these out:
If you are new, go for the PADI courses and get your certificates first. Then in future, you can go for leisure diving.
Cost depends on courses as well as the location you are diving.
Beware of hidden cost.
Sometime, there maybe promotion gimmicks where it stated a price, but in the end, you still have to pay extra for the diving equipments, rental of boats, food... Then you end up paying much more - unwillingly ..
as you only find out abt it when you r already in that island, preparing to dive!
You can check what does the price/cost includes. Normally they should quote you the final cost for beginners.