I got certified on Koh Tao. There are a lot of places that will offer certification, and they will throw in extras to get your business.
There are a lot of scuba diving places on Koh Tao. I got certified with Dive Point
. The place is owned by a German (Austrian) guy. My class consisted of myself and my traveling partner at the time. We were taught by a Thai lady that spoke excellent English. I was satisfied with the experience. (They also had the best dive boat, imo.)
If you like being herded with the rest of the young backpacking types, there's always Ban's and Crystal's. A lot of the people on scubaboard will also vouch for Master Divers.
When you get off the boat (or even on the boat), people will hand out flyers about scuba diving. Be sure to have a list of dive shops you want to check out and ask questions, like "How many students per class?", "How much?", etc.