Identify Obvious Keywords - the ones you can think of easily, often this will include your products or services.
Look at your websites access log for keywords that are bringing people to you.
Look at your competitors keywords.
Look for other additions: common spelling mistakes, synonyms, split or merged words, singular or plural, hyphenated words, geospecific words, your company name.
Use a keyword tool like GoogleAdwords Keyword Tool or HubSpot Keyword Grader to find more relevant keywords and keyword phrases.
The goal is to bring more traffic to your site and then have more of those visitors continue through the conversion process to become leads and then customers. The goal is not to just rank highly for as many keywords and keyword phrases as possible. So how do you know if this is happening?