If you follow the link below, you will see that the first step to writing a good introduction is to do your research. You cannot write an introduction until you know what you are going to say in your essay. You do not start by writing the intro. You start by figuring out what you are going to say.
So, what your introduction says completely depends on what you are going to say in your essay. One thing you might talk about is why the use of ecstasy is declining. In such a case, your introduction would need to set out the basics of your argument. You might say something like:
Why would a hip teen be seen dancing and sucking on a pacifier? Maybe because the pacifier has drugs on it. Ecstasy, the drug on the pacifier, has been a very popular drug. However, the use of this drug has been declining in recent years. Why might this be? The best answer for this is... (and here you tell what you think is going on).