Hi there!
It really depends on your priorities for your holiday. Is getting your licence your main priority? Or is it more the location in terms of sightseeing, beachtime, nightlife, other activities etc whilst there? There are so many excellent options for diving in Thailand its difficult to narrow it down without a few parameters!
On the west coast you have the island options of Phuket, Koh Phi Phi and Koh Lanta (Khao Lak season will be finished so that is out of the equation), and mainland options such as Krabi/Ao Nang.
To the East in the Gulf of Thailand you'l find Koh Phangan, Samui Tao and Koh Tao, of which Koh Tao is the better option (dive operators from the other 2 generally just charge more and speed boat you to Koh Tao dive sites anyway!).
Further north and closer to Bangkok you can dive on Koh Samet, and around Pattaya - although personally we wouldn't like to spend too much time in Pattaya if we could help it! There is also Koh Chang a little further East.
Once you've narrowed down your geographical area, we'd recommend having a look on trip advisor under the activities section for your chosen location, where you can find reviews of all the dive centres - and other activities - in the area, all written by previous customers. More and more travellers to Asia are now using this as a planning tool to get the most out of their holiday. From that you can build a shortlist of schools and either make email inquiries, or visit in person on arrival to get a face to face impression, although this approach may mean some courses are already booked up if you have not made a prior reservation.
One other thing to bear in mind...April 13th is Songkran (Thai New Year) and as such can be a very busy time of year for tourism, so its definitely worth booking transport, accommodation and your dive course in advance if possible. And we'd recommend not doing any travel portions of your trip on the 13th itself. Songkran is a water festival, but over the years has pretty much just turned into one big water fight, with anything you wear/own that you have with you getting saturated by water ballooons, supersoakers and general drenchings! Its great fun, and an excellent day to experience, but leave anything valuable - especially electronics - at home! Most smaller locations celebrate Songkran for just one day, but in some locations it can go on for a few days to a week. So be sure to check with your dive operator/hotel etc so you can be fully prepared beforehand!
Hope this helps. Safe travels and happy holidays
