A successful opening line has two characteristics:

Your next lines must provide an essay map, i.e. tell the reader what avenues you have selected to explore to answer the question, and why you have chosen these avenues and not others.

By the end of your first paragraph, though you may well not want to reveal your conclusions, your reader should know what topics will be discussed.

One very big tip: whether the essay title is long or short, read it from start to finish, then read it again. Underline key words. If you do not read and understand the title properly, you cannot write a relevant essay. Irrelevance loses marks. Learning to be relevant is key to both a good degree and success in the outside world. University essays offer and excellent opportunity to hone that skill.