what type of boat,and where to start??
There are two ways to go here but I think as an adult it is easier to start sailing on a light weight 22 to 25 foot fin keel boat. This size is small enough to be manhandled and yet big enough that you can concentrate on learning individual skills. I think that the boat should be responsive enough that you can feel and understand the effects of sail trim, boat handling, weight distribution, and the other factors that are part of sailing.
It could also be argued that Dinghies are a better venue to learn to sail because every action has such a direct reaction.
I spent a part of last weekend teaching adults to sail in dinghies after normally teaching people to sail on small keel boats. I found that there was almost a sensory overload in sailing a dinghy because so many things had to be done at once, that it seemed harder to teach the invidual skills before having to put them all together.
Beyond all of that a lot of people have learned to sail to one degree or another in bigger boats but I personally think that if you want to really learn to sail well it is a much more difficult process in boats over 25 or so feet.