How do I choose the essay I want to do for the LNAT?

So, you get an option of 5 essays when you do the LNAT. The first thing to do is to skim all of them. Immediately, you'll be able to discount one or two - maybe they sound boring, maybe you've never heard about the topic. That leaves us with around 3 more, from which you can choose one essay to write.
Remember, you only have 40 minutes to form an argument and write it persuasively. If one of the questions is interesting, but is more suited as the topic of a book rather than an essay (i.e. feels too complicated to cover in 40 mins), don't do it. The same goes for topics that don't spark any passion, even if you know lots about them: you're not going to be able to write persuasively about something you don't care about at all. The best essay question is always going to be the one that makes you go "oh, I have an opinion about that!", and as long as you've been reading the news and keeping up to date with common debate topics (nuclear weapons, medical ethics e.g. euthanasia, gun control, embargoes etc) there will most certainly be a question you can answer. Keep calm and make sure to a) plan and b) deal with any opposing arguments: the most persuasive answers always say why any opposing opinions are wrong!