I'd like to ask Japanese about proper way of using formal and informal language.
I always get confused when I need to speak politely and formerly. I'm used to using informal language and I don't get pleasure from using -ます、　－です。 But I realize I should be carefull not to offend native speakers, because I think Japanese are used to some level of politeness with strangers and they will feel uneasy when somebody's careless about it, even if it's a foreigner. You know, there's a similar thing in Russian: there are two forms of "you": a formal and an informal (used with friends or closer acquaintances). This is not so complicated as in Japanese, but still one has to use a polite form of "you" with a stranger. And sometimes when foreigners don't, I feel strange, even though I understand they're just used to informal forms or they want to practise informal language, still it takes me some time to get used to saying informal "you" to an unknown person. So... Do you get offended or do you feel wrong when foreigners make mistakes using wrong forms, not that polite they're supposed to use? What are the most often mistakes in levels of politeness that you see when strangers speak Japanese to you? You can write your answer in Japanese, I know basic Japanese and I can understand quite much, I'm just lazy to write such a long article in Japanese. :)
I have a couple of questions more about this:
1) How much time should pass untill I can call a person by name, not using -san? I feel a little uneasy using -san form of the name, that seems too official to me, but they keep using it, so it must be necessary.
2) Why is one person using both -desu and -da talking to me, the same person? It must be that way, there are some cases when you can use only -desu or only -da (sounds impossible)? Or they just can't decide whether they can or can't speak informal to me?  
3) And I'm so sad that since I'm a girl, I can't use 「よ！」 at the end of the sentence,  「ほい！」とか、　「行くぞ」とか、　「嘘！」とか。。。　この言葉を大好きです、でも失礼ですね。。。 悲しいです。。。　Ｔ＿Ｔ　This and also ways to address elders and youngers is the only thing I can't accept in Japanese, it makes me sad. :(
I'd like to know native speakers' opinion on this issue, please only native speakers, only Japanese please!