You'll see them jostling for position even before you're off the ferry. A relentless horde of instructors, divemasters and assorted hangers-on waiting for you with a carefully-pressured sales pitch about why their dive shop is the only safe/fun/affordable option. Competition between the schools is cutthroat and this nasty spectacle gets uglier every year.
The truth is, all dive shops offer a different environment, and there's no way to know right-off-the-bat which one is right for you. Don't listen to anyone that tells you you have to sign up now, or that slanders the competition. All of the island's dive shops are safe and fun. In this writer's experience, negative reviews are incredibly rare.
Many places will offer you a $3 dorm-room with no strings attached for your first night. A good idea is to take one of them up on it, drop off your bags, and spend the first day exploring different shops. Don't let them pressure you into anything. Look through at least a few different places before you make your choice – it's an important decision, since your dive shop is usually also your home and your social scene. Make sure to see their accommodations and find out exactly what you're going to get for your money. If you're desperate to start right away, another option is to drop your bags off at Captain Morgan's Dive Shop at the crossroads. They'll watch your stuff while you do some comparison shopping.