My own belief is that a somewhat tired old boat is a good first one. It will teach you something about maintenance, and it will let you take some risks as you're learning--scratching an already-scratched hull is far more tolerable than scratching a brand-new one.
In general, a boat that can be trailered and handled by one person is probably best; you'll sail lots more if you don't have to get a friend to help out.
Sailing clubs can be a great way to learn. (jh)