Savapet
You need to do a little self evaluation. If you want to run around a trampoline like a couple of monkeys and go for an occasional swim , then a bleachable cat is a good choice. Small center boarders would also meet your needs. If you would rather kick back on a sedate cruse, then a small keel boat would be more appropriate. Since boats in general have a substantial depletion rate, I'll assume you will be looking at a used boat.
General:
Condition, condition, condition. For a small, boat the cost of repairs can easily exceed the value of the boat. Used boats may be cheap, but replacement parts aren't. Be especially careful to check the sails. Have the seller hoist them for you. Since you will be looking at salt water boats. Be mindful of metal fittings.
Being popular is a good thing. Popular boats are numerous for a reason. Builders like Hobie and Catalina are popular because they make good boats. Not that others don't, but novices will have a hard time deterring build quality. Also, popular (especially locally popular) boats will give you a better population to choose from. In addition, there will be tons of information on the internet for popular boats.
One stop shop. Do your best, to get everything you need when purchasing the boat. Don't buy a nice boat and then start looking for a trailer. Since all the equipment depreciates, try and get the boat, trailer, PFDs, horn ... from the seller. It will be much cheaper than buying that stuff new.
KISS. Keep the first boat simple. Boats with just a main and jib will give you plenty of experience. Focus on getting those trimmed right and you will be doing fine.
Catamarans:
Fast, fun, easy to beach, rugged, high initial stability (stays flat), very responsive, easy to tell when it's trimmed good, most unsinkable, very popular.
Relatively hard to tack because of it's low inertia. Most stable position is upside down. Your time on the water will be determined by how much exposure your body can take. Not all of the experience translates to monohulls. Will take up lots of garage space.
Dagger board monohulls:
Fast, fun, easy to beach, low initial stability, very responsive, easy to tell when it's trimmed well, some unsinkable, popular. Least garage space.
Relatively easy to tack. Almost no stable position. You will be constantly moving. Your time on the water will be determined by how much exposure you body can take and your endurance moving from side to side.
Keel boats:
Slow, sedate, stable, easy to tack because of high inertia. Not as responsive as lighter boats and a little harder to tell when trim is good, because the response to trim change slower. Generally not as popular as small daggers or cats. Some have a cubby cabin for protection from the elements.
