I have had more than 20 APs over two decades. You sounds like a very thoughtful person and I am sure you will make a great AP. Good that you're asking advice.
FWIW I think other people have covered much of the same ground as I would have done, but I would add it's important to add that the best APs behave as an adult in the relationship, and not see the thing as a kind of adoption or parent/child thing. I see a lot of really alarming profiles on APWorld that go on and on about 'my new family' and so on, as if APs seriously expect this, and I have certainly had a few young women in their 20s work here who were expecting me to swoop them up and mollycoddle them and look after them, as well as do their duties for them, and ended up being sadly disappointed because I refused to be a surrogate mum on those terms. I also moan about it on MN, btw, because it's depressing and tiring when this happens.
A good host family will always be kind and helpful if you have your handbag stolen, get the flu or lose your boyfriend, but as far as day to day behaviour is concerned, you need to be reasonably independent of spirit and stand on your own two feet a bit.
Essentially you are there to provide a service - usually childcare - and the fact that you get some support with lifts to and from the airport, occasional family meals, the odd tenner slipped to you for a taxi before a big night out, and inclusion in occasional sightseeing trips is the limit of what most families will expect, and what marks it out as an AP relationship and not a domestic staff one. Over time if you get on with the family well they will probably do more for you, for example we have had an entire AP's family over to stay for a week recently (and actually all had a great time).
The best APs use their initiative a lot and try to understand the mindset of the host family, how tired the parents get and so on, and appear magically with sleeves rolled up ready to help take the strain. Good APs say things like "You look tired, have you had a hard day at work? Can I make you a tea?" and "I had fun with the children today. Look at what we made" and "Let me fold that washing. You get your feet up" and "Shall I take the kids down the park so they can burn off some energy?" Rubbish APs say things like "I don't want to collect the children from school because it's raining and I don't have a coat, can't you do it?" and "Sorry, we have used up all the milk making milkshakes, can't you go back out and get some more?" and "When you are at work, your children behave very badly and you need to do something about it because it's nothing to do with me" and "I have broken the hoover again because I was trying to hoover up mashed potato". (These are all RL things various APs have said over the years to me, by the way).
So this is a very long way of saying that the more adult you are, the better it will all go. As far as preparing is concerned, it might be useful to read 1-2-3 Magic by Thomas Phelan, which gives you some insight into how children try to wind people up, and what you can do about it. It might also be good to learn to cook a few simple meals and tackle washing and ironing on a family scale.
Best of luck.