There are two certification courses for beginners. Scuba Diver and Open Water Scuba Diver. The first combines a limited amount of theory with 3 pool sessions and 2 dives in open water (these are actually the first sessions of the Open Water Scuba Diver programme). After certification you are allowed to dive to a depth of maximum 12 metres and are required to be guided by a diving professional.
The Open Water Scuba Diver level allows you to dive with any certified diver. You do not depend on a professional. The theory is more substantial and there are 5 pool sessions and 4 dives in open water. Those who already hold a Scuba Diver certification do the whole theory for this course, but only the last two pool sessions and the last two dives in open water (the other sessions were part of the Scuba Diver programme).
You want to try it first? The Try Scuba programme allows you to try breathing underwater in a swimming pool. If you prefer to go in the open water, then the Try Scuba Diving programme is your choice. Participation in the Try Scuba programme is possible if you are 8 years or older. Try Scuba Diving requires a minimum age of 10.