It will depend on the situation and what tone of voice is needed. If you are angry you would stress certain words but not others.

It really depends on the point you try to make. John gave me the book - JOHN gave me the book John GAVE me the book John gave ME the book John gave me the BOOK all emphasise something different. Just keep listening, and keep copying, keep practising, it takes time. :)

In general grammatical words are unstressed (except when you really want to emphasize one of them). If you're asking about the general position of stress within words (as opposed to within a sentence), there are rules for stress placement, the most general (even if there are some minor rules saying otherwise) are : words of two syllables are generally stressed on their first syllable, words of three or more syllables are stressed on the antepenultimate syllable.