In fact both are really important to be well understood...
In words of more than one syllable, there will be one syllable which is more heavily stressed than the others. We say that this syllable carries the primary stress (eg. apartment). In some longer words, other syllables may carry secondary stress. The most important thing is to recognise where the primary stress is - and remember that it can change between words derived from the same base (eg. photograph, photographer, photographic). If you stress a word incorrectly, it can be very difficult to understand, so it is important to learn how a word is stressed at the same time as you learn how to pronounce it. If your native language does not have a word stress pattern like that of English, you will need to be especially conscientious in learning the English forms!
* When you pronounce new words, exaggerate the stressed syllables until it becomes natural for you to put the stress in the right place.
Intonation is the name given to sentence stress, or what is sometimes called the "music of the language". Just as words have stressed syllables, sentences contain regular patterns of stressed words. In addition, the voice tends to rise, fall or remain flat in various different types of phrases and sentences. You will need to pay attention to intonation if your native language has different intonation patterns from English, and especially if you normally use a flat intonation - in English, this tends to signify boredom or sarcasm!
* Listen as carefully as possible to the intonation patterns of native speakers, and try to copy them.
* If you find it hard to hear the intonation patterns, ask your teacher to give you some examples.
* Ask your teacher whether there are particular areas where you have difficulty with intonation, and focus on those areas.