Thailand: first ever dive info/advice

I'll be in Thailand for the first 2 weeks of October. I've never dived
before whatsoever and wondered if this would be a good thing to try out
while in Thailand.
Clearly I won't have the expertise to assess any given dive shop, if
their gear is any good etc, and I don't know where to dive, nor what to
look out for with weather and sea conditions.

I don't want to try and get any qualification, just have a go.

Being optimistic: going for a couple of days diving with a reputable
dive shop should be easy, safe and fun. (any recommendations?)

Being pessimistic: this is a bit stupid - I ought to have a vague idea
what I'm doing before I think about giving it a go.

Any advice? (including if I ought to have posted this elsewhere!)

Cheers,
rc