That's a good question and everybody wants to know the answer to it. If you choose a high paying keyword to focus on like lemon law, depression or nintendo, you have to write really excellent content and lots of it, then build hundreds, maybe even thousands of links to beat your competition and the competition is huge. Focusing on low paying keywords with a high search volume may be a better option.
You should think about what you would like to write about, ask yourself how much you know about the topics you choose and are you ready to learn more, if you don't feel like you know enough. Make a list of keywords associated with the topic you are interested in and do some research using the Google keyword tool or Wordtracker (or some other keyword tool) to see how many users search for information using those keywords and what the cost per click for those keywords is.
A hubber known as Susana S has a nice hub about web content and what topics to write about to make money. It may help :)