You need to consider very carefully which words and phrases people are likely to use in searching for the service you offer or the product you sell. This requires imagining that you are a potential customer who has never heard of you or your product before. Ask yourself: "what would I enter in a search engine if I was in their position?" The terms you might enter are the words and phrases you will need to consider building into your site.
Having worked out which words and phrases by which you hope people may find your business, you must then assess how much competition there is for these words and phrases.
However, it is not necessary to spend a whole lot of time seeking for words and phrases for which there is little competition. The most productive use of time is to put as many pages out on as many relevant words and phrases as possible, and wait to see how well they do. Those pages which do reasonably well can then be modified further until they reach the highest position reasonably achievable. This trial-and-error method is actually a better use of time, because researching "keyword"-competition can be so time-consuming and is highly unreliable, unless carried out by an expert.