Look at the opening of the set question:
There are more and more old people in many places of the world. What are the causes of longer life span
Look at your opening:
In many parts of the world, especially in some developed countries,(so,"more and more" - people have a longer life expectancy -(a longer life span).
1. You've launched straight into the essay - this is not an Introduction
2. You are merely reiterating (yawn) the question.
Let's tackle this my having a look at the first topic:
In many places of travel such as lakes, mountains and beaches, the environment is polluted because of the rubbish and litter produced by tourists. Why is this? How can we solve the problem?
So - the question is asking (i) why are tourists polluting (ii) knowing why, how can we address this, solve it.
Your Introduction, then, is to set a framework for what you are going to say.
Your first task in writing this essay, then, is to make a list of one or three word headings, answering 'why'.
Can you come up with three reasons -just a few words indicating each - why this is happening?
If you want to do that, I'll check back.