Hartley's got it just about right. If you learn on a centerboard boat, you'll have no problems with a keelboat. I put in quite a few days on an Alberg Typhoon (mentione above) and loved it, but a lighter-weight centerboard boat will teach you more about what makes a boat go (and stop) than a keelboat will, so start with whatever decent centerboarder is plentiful in your area.
Then move up to a keelboat. You won't regret having started with a smaller boat, that's how the best sailors are built.