Most people choose their first dive shop by comparing location, course prices, and schedules. That's understandable when you haven't learned enough about diving to judge the training itself. But the instructor who teaches your first course will shape habits you may carry for years, and I would put considerably more weight on that choice than on convenience or the logo printed on your certification card.
You're choosing two things: a shop that provides the training environment and an instructor who teaches you to dive. It's worth evaluating them separately.
Start With the Shop
Before paying for a course, ask how much pool time is included, where the open-water dives take place, and what happens if weather cancels a training day. Find out what's included in the advertised price, from learning materials and certification fees to equipment rental and open-water expenses. The cheapest advertised course may not be the least expensive once everything is counted.
I would also look at the rental equipment. Regulators and BCDs don't have to be new, and ordinary scratches don't worry me. I want equipment that appears properly maintained and a shop that can explain its servicing and replacement procedures. A beautiful showroom is less persuasive than straightforward answers about the gear students actually use.
Pay attention to how the staff responds to questions. If they're impatient before you've enrolled, I wouldn't expect that to improve once they've taken your money.
Class Size Changes the Experience
Personally, two students per instructor is ideal, and four is about the maximum I'd want in an introductory class. I would be reluctant to enroll in a course with eight students assigned to one instructor. That isn't a claim that larger classes automatically violate training standards or are unsafe. It's a judgment about how much individual attention a new diver deserves.
Consider mask clearing. One student understands it immediately; another needs several attempts; a third is frightened by water around their nose. The instructor has to recognize those differences and respond to them while supervising everyone else. The larger the group, the harder it becomes to devote time to any one person.
Ask how many students will be in the water with the instructor, not just how many registered for the course. Divemasters and other assistants can help with supervision, equipment, and logistics, but they don't remove the instructor's responsibility for teaching and evaluating students.
Talk to the Instructor Who Will Actually Teach You
Training agencies set requirements and minimum standards. Those are important, but they don't tell you whether the individual instructor is patient, observant, or good at explaining unfamiliar skills. I've discussed agency differences in Choosing a Dive Agency. For your first course, I'd choose an excellent instructor over my preferred agency.
Ask to speak with the person assigned to your class. I would want to know how they teach students who learn at different speeds, how they handle someone uncomfortable with a skill, and whether they can explain their approach without reciting a list of certifications. You're not qualified yet to judge their underwater technique, but you can judge whether they listen to your concerns and answer your questions.
Most importantly, ask what happens if you aren't ready to move forward on schedule. Perhaps mask removal takes longer than expected, or you still struggle to control your buoyancy. Will there be extra pool time? Can you repeat a session? What will it cost? Those answers matter more before you enroll than when you're already halfway through a class.
Certification should require demonstrating the skills, not merely attending the sessions. At the same time, students don't necessarily become competent on the same timetable. A good program can explain how it handles both realities.
Consider Private or Semi-Private Training
If your budget permits it, I think private or semi-private instruction deserves serious consideration. More individual attention means less waiting while other students practice and more time spent addressing your particular difficulties. That can be especially valuable if you're nervous, unfamiliar with the water, or simply learn better without a group watching.
A small class with a good instructor can also be excellent. My preference isn't for private training at any cost; it's for a reasonable amount of instructor attention and enough time to learn.
The outcome I care about is a diver who can perform the skills rather than merely hold a certification card. Find an instructor who will give you the attention and practice time to get there, and a shop whose training arrangements support that work.