Given you have a good idea of where to start and are fairly confident you are speaking the same language as your client, jump start research by generating related keyphrases and long tail variants with the ever so easy to use Google Autocomplete. This tool makes predictions based on what you are typing that are a reflection of Google search activity.
Are you a business owner, online marketer or content creator? If so, most likely you would like more people to visit your website, read your content and buy your products or services. The easiest way to achieve it is to find out what your potential customers or readers are searching for on Google and create content on your website around these topics.