To a certain extent, your first boat might be dictated by what is available in your area. I started sailing in an area where sailing wasn't very popular, so the choice of sailboats was very limited.
In addition to those already mentioned, another good, basic sloop is a 16 foot Ray Greene "Rebel." The Rebel is often reputed to be the first fiberglass sailboat, built in 1948. It's lively enough to be fun but forgiving enough so you'll only take a swim if you really mess up.