My advice to you, OP, at this point would be to start gathering nice people you find in an LS or FC, or join one that suits that. When it comes to Duty Finder, you never know who or what you're going to get. Some people, unfortunately, have a chip on their shoulder about newer players. By finding people you can actually have fun with and enjoy the game with, you will have a much more pleasant experience, especially once you reach lvl50.

Use the tools at your disposal; check the Server Forums here, ask around in Party Finer, in shout, anything that will help you find people who either share your exact goal or are willing to help you get there. This will be important when you decide you're ready to take on things like The Binding Coil of Bahamut or other high level dungeons. Also, Castrum Meridianum and Praetorium will be a much more pleasant experience if you don't use Duty Finder at all. It may take longer to find at least 8 people who will join you, but the experience and friendship you gain from it will be invaluable.

As far as actual dungeons themselves go, there are 2 schools of thought: those that would prefer to go in blind and learn through practice/experience, and those that prefer to look up everything they can about it beforehand. This sparks tons of debates, and in reality both schools have valid points. Think about yourself, and what you like to do, and try to find people who will share that idea. Have a Goal and Vision in mind. Either direction takes practice, there's next to no avoiding that unless you are a prodigy, but how you handle the failures and pick yourself up are what's important. Good luck, and I hope you find what you're looking for.