The first line is one of the hardest to write in almost anything, in my experience. Often better to actually write it after you have gone through a good part of the rest of the essay. But when getting started with the actual story that you will be telling, (getting into the actual flow of the text), it is often best to begin at the beginning (sounds simple enough right?). Come back and create an intro afterward when the point and the flow of the text has become a lot clearer to you.
That is how I work, anyway. I don't know the details of what I will write until I actually write it. I know very well the broad idea, but the process of actually writing something gives the broad idea a structure and direction that is not basic to the broad idea. Things get left out, other things pulled in, and that is not exactly what the original idea had in mind.