How to know which words to be stressed in a sentence?

I'm a chinese in Hong Kong.
First of all, non-native english speakers in HK speak their English with many local accents, even teachers in secondary school would make so many mistakes.

Here are some examples:
V sounds pronounced as W
th sounds pronounced as f (thanks->fans)
K sounds missed in ex- es- etc.
Voiced th sounds pronounced as t sounds
You can seldom distinguish Feel and Fill
Never know cotton/button/shouldn't/couldn't are more frequently pronounced with no voiced schwa
.
.
.
Sorry for some non-native expressions, I'd like to hear you guys advices in improving my English. I believe that most of the time my sentences are grammatically correct.

Ok, my question is I can't understand which words in a sentence should be stressed.

For example, I am a boy.
I know under what situation should I stress boy or am, but in many more complicate sentences, it's difficult to tell.

Are there some clear clues that can help me understand?

Thanks for your kind help.

Proofreading : complicate->complicated

I agree with Josh: it's a difficult subject. Books usually don't discuss this sort of thing because it's difficult to convey this sort of stress through text, and different people have slightly different patterns of stress. However, a good rule for any language with stress is that words like articles, prepositions, and conjunctions -- the "function words" -- are never stressed unless they're being emphasized. Pronouns might be stressed a little, but not as much as other words unless you're emphasizing them.

The best thing to do is just get lots and lots of practice listening to native speakers. If you're still having trouble figuring out what native speakers are saying, you should probably handle that problem first. But if you can easily understand what they're saying, pay more attention to how they're saying it. Since you speak Chinese, I imagine it'll be easy to notice the changes in pitch. The good news is that pitch and stress are often related in English, so that should help you listen for stress as well.

- Kef

There's an example in Wikipedia with the sentence "I didn't steal that money", and what happens to the meaning when a different word is stressed, I'll look for it and I'll post it here if needed.
Basically it says that:
stressing 'I' may mean somebody else stole it
stressing 'steal' may mean you got it in a different way
stressing 'that'may mean you stole some other money
stressing 'money' may mean you stole something else.
A native speaker or a wikiaddict may be able to expand/correct what I've just said.

That's correct, Rodrigo. Also, stress on "didn't" emphasizes that you absolutely did not steal it -- usually in response to an accusation that you did.

- Kef

I can understand what native speakers are saying despite some words that I don't know. But for those who had strange accent I almost totally lost.