I've heard so many bady experiences! How do I score a nice family?

Hello! My name is Casey, I'm from Australia and am considering becoming an Au Pair in America but I've heard so many bad stories from friends that I only want to do this if I'm confident that I'm going to be placed with a genuinely nice family, I would love some help with how I can assure good potential families that I am would be good for them.
1st: I will be 18 in Feb 2010, how can I assure my potential families that I'm not like most typical young women and will not drink like a fish and go out partying all the time? I understand parents are reluctant to want someone that hasn’t been on the roads very long, how can I assure parents that I am a really safe, good driver?
2nd: I don’t speak any other languages and there isn't a huge culture difference between Australians and Americans, do you have any ideas to how I can stand out?
3rd: I am not heavily religious but I have some Buddhist values and am Christian orientated, is this something I should be really open about or better to not talk about religious beliefs within the first stages of interviews?
Any help would be greatly appreciated!
Thanks,
Casey.