The first thing you will need to consider is what sort of sailing you wish to do. Your location will dictate some of this to start with. You are more likely to take up dinghy sailing in the midlands than you are offshore yachting for example.
The first thing to do is to find your local yacht club, reservoir, sailing school or friend with a boat. Once you have done this you can get some advice on how to proceed. If you can get a trip out with a friend of a friendly face from the sailing club then go for it. The more exposure to different types of sailing you get the easier it will be to narrow down what you want to get out of it. The best way to proceed after this is to look for a course to enrol on. Bear in mind that any business will be slightly biased to offering you their course. If you want some impartial advice then the best bet will be to call the RYA and ask them for a list of local venues and centres for you to start at.
Any RYA syllabus course will give you a basic level of understanding of the sport. They have courses tailored to dinghy, multihull, keelboat racing and cruising. The most important part of this course is to keep you safe. As well as knowing when to duck there are certain things that can can carry/wear that will not only raise your level of enjoyment by keeping you warm and dry but will also help keep you safe in the unlikely event of an accident.