You don't need to know how to dive before your first class. Your instructor will introduce the equipment, teach you to manage yourself underwater, and help you develop the skills required for certification. Most Open Water courses follow three stages: academic learning, confined-water practice, and four dives in open water.
Before Your First Class
You'll complete a medical questionnaire and demonstrate basic swimming ability, typically swimming a specified distance and staying afloat for ten minutes. Certain medical responses may require a physician's evaluation before training begins. You don't need to be a competitive swimmer, but you should be comfortable in the water.
Most shops expect you to bring a mask and fins, and some require a snorkel. Regulators, BCDs, cylinders, and other equipment are generally provided or rented. Ask what's included before buying anything, and if you haven't enrolled yet, see How to Choose a Dive Shop and Instructor.
Learning the Fundamentals
The academic portion covers pressure, equalization, gas consumption, equipment, dive planning, and the procedures divers use to communicate and manage problems. Much of this work is completed online, although some programs include classroom instruction. You'll complete knowledge reviews and assessments to demonstrate that you understand the material.
Some concepts will be unfamiliar, particularly how depth changes the pressure around you and the gas you consume. Your instructor should explain the reasoning rather than ask you to memorize answers you don't understand.
Your Pool Sessions
Confined-water training usually takes place in a swimming pool or similarly controlled setting. You'll assemble and check equipment, practice breathing through and recovering a regulator, clear a flooded mask, share gas, communicate with a buddy, and make controlled descents and ascents. You'll also learn to manage buoyancy, something that rarely feels natural on the first attempt.
Mask clearing can be especially uncomfortable because you're deliberately allowing water around your face while continuing to breathe through the regulator. Other students may find the mask easy but struggle to stay at one depth without kicking. Different skills come easily to different people, and that's why pool time matters.
If you need additional practice, tell your instructor. A good course requires you to demonstrate the skill, not simply try it once and move on because the schedule says so. Extra sessions may mean additional time or expense, but those arrangements are worth making rather than entering open water with a skill you cannot reliably perform.
Your Four Open Water Dives
After completing the required academic and confined-water work, you'll make four certification dives, generally over at least two days. The location might be an ocean, lake, or quarry, and conditions may be less forgiving than those in the pool. Visibility, temperature, entries, and exits all require more attention when you're wearing unfamiliar equipment.
These dives aren't simply guided sightseeing trips. You'll demonstrate skills practiced in the pool while beginning to manage an actual dive: checking gas, staying with your buddy, following the plan, and making controlled descents and ascents. If your ears won't equalize, stop the descent and address the problem rather than forcing it.
Some students settle into open water quickly; others need longer before the equipment and procedures feel routine. Your instructor's job is to evaluate whether you can perform the required skills, not to hurry everyone through four dives.
What Certification Means
After successfully completing the required training, you'll receive an Open Water certification. A typical adult recreational certification allows you to dive with a buddy within the limits of your training, commonly to 60 feet (18 meters). Age, certification, experience, and conditions may impose shallower limits.
Your certification means you've demonstrated the entry-level skills, not that every dive will now feel easy. I'd spend time making uncomplicated dives, learning my equipment, and improving buoyancy before rushing into additional courses. What to Do After Open Water Certification discusses that progression, and I describe the training experience more personally in The First Breath.