The following question confuses many students because it doesn't seem to be one of the 3 normal types (opinion, discussion, problem/solution).
Happiness is considered very important in life.
Why is it difficult to define?
What factors are important in achieving happiness?
I call this a "2 part" question, and it's easier than you think. Just answer the first question in paragraph 2, and the second question in paragraph 3:
1. Introduction - topic + simple answer to both questions
2. Paragraph - explain why happiness is difficult to define
Some ideas: Explain that happiness means different things to different people; the same situation can make one person happy but another person sad/bored etc. Give examples of what makes different people happy (money, success, family, friends, hobbies). Give an example from your experience e.g. a hobby that makes you happy but that your friend hates.
3. Paragraph - give your opinions about how to achieve happiness
Some ideas: Try to organise your ideas logically - you could begin with humans' basic needs (in order to be happy we need to be healthy and have somewhere to live, food to eat etc.) then talk about the importance of family and friends. You could then write about career, money, success, status etc. - maybe you could explain that these things mean nothing if we do not have family and friends to share our experiences with. Give an example from your own life - what will make you happier than you are now?
4. Conclusion - repeat/summarise the main ideas