There are five things to consider when buying and owning a sailing dinghy:
The factors involved in choosing a boat are many. There is a vast number of dinghy classes to choose from, so you might as well narrow things down by looking at where you are going to sail, which will probably be your nearest sailing club. Although most clubs (but not all!) will let you sail anything suited to their water, it's more fun to sail a class which has a good following at that club, so you will have a ready source of hints, tips, spares, chat and others to race against.
How many people will you want in your boat? Two handers are more sociable and allow you to share all expenses and effort, single-handers mean you go sailing when you want. If you're considering a two-hander, you need to ensure that your proposed partner wants to sail approximately the same amount that you do, whether that is four times a week or once a month. It is always possible to find another partner, but you might as well start off on the right foot. Some boats such as Mirrors and Enterprises (in suitable conditions) can be sailed by one or two people.
Also consider what else you may use your boat for. For example will you want to take it with you on family holidays or perhaps spend a day cruising in the Lake District ? If so it will be of little use buying a Laser.
Choice of boat will also depend on experience. Some dinghies are easier to sail than others. Whilst the author was learning to sail he and his partner ventured out on a blustery day in the sailing clubs' own Lark. The lark is quite a powerful boat which can be a bit of a handful in strong wind. Suffice to say the Lark remained upright for less than 5 minutes before ejecting both the author and his partner overboard and then capsizing. Learning by his mistakes the author bought a secondhand Mirror and never looked back ! Forget about sleek looking RS200's and 400's whilst learning to sail. There is a thriving second-hand market in all the established classes, so you can always trade up.
Consider how much maintenance do you want to do. Remember wooden boats need loving care whilst fibreglass boats need some attention, but much less.
How much do you want to spend ? There is no need to buy a new boat; even quite elderly examples can be perfectly sound and speedy. The author started out with a £50 Mirror dinghy. Remember when you are learning to sail you WILL make mistakes and perhaps damage the boat. It's better you make these using an older, less expensive boats than a £3000+ one.
How often will you want to sail ? There is no point buying an expensive dinghy which will only get sailed twice a year. Make an honest assessment of this when considering a purchase.
Somewhere to sail means any patch of water, it is most fun if it is at a sailing club. These are found on reservoirs, lakes, rivers and on the coast, so check out your area and visit clubs. Most welcome new members with open arms but its best to spend a bit of time at each one in your area. Many sailing clubs which, in the past have been heavily geared towards racing are now starting to realise that to attract new members they must provide some sort of training. Also check things like sailing times, what sort of boats they sail etc. Membership costs vary depending on location, facilities, opening times and whether there are any staff.
Somewhere to store the boat may well be catered for by berthing provided at your club. You will need to get the boat there, and you may well want to move your boat to other race meetings or for holidays. Small single-handers can be car-topped, but bigger dinghies require a trailer and tow-hitch on the car. Some clubs may only provide berthing during the sailing season so you may have to store the boat elsewhere.
Clothing starts with a pair of shoes that don't mind the water, a swimsuit and a buoyancy aid - this is compulsory at many clubs and a good idea for everyone. As you get into the sport and sail in bigger winds and cooler temperatures, you'll need to increase the wardrobe with wetsuit, dinghy boots, gloves, spraysuit and a host of other paraphernalia, but there's no need to buy it all straight away and much is available second-hand or cheap. If you get sufficiently hooked to sail through the winter (it is possible) you will need a good wetsuit or drysuit. Note that clothing for dinghies is very different from clothing for yachts.
Insurance covers not just fire and theft, but collision. Dinghies capsize, winds change and things break, and collisions are not unknown. Usually they just result in a few scratches to paintwork, but it is possible to do more severe damage and hence you must be covered against financial consequences, both to boats and people. Racing means that collisions are almost inevitable in the heat of the moment, and so most clubs will not let you race un-insured. Premiums start from £20 per year for a small single-hander.