You have to look at the audience you want to bring to your website. Try to see the terms your competitors use, that would work for visitors. Remember that single words tend to be more competitive. So you may need to find two and three word phrases that would work for a searcher looking for your website topic.
Importantly, make sure to focus your terms on the actual topic of your website, and terms that people would really search for. Think like a searcher - not a website owner.
Viewing your competitor's source code is very easy and a good way to see what keyword phrases (if any) they are using. Using your browser, view the source code of their page. The title and meta tags should contain the same keywords or variations of keyword phrases if the competitor's website is optimised. Look over the web page content as well as for keyword phrases worked into the text, headings and hyperlinks of the pages.
You can use some keyword suggestion tool to find key word and phrases and related word and phrase to them. Clicking on the suggestion tool link will bring up a window that allows you to search for terms and variations of terms. Begin with your list and see how many variations come up with the results. You might be surprised at the popularity of some of the search variations you see. Be sure to add you new keyword phrases to your list. Trellian has trial you can use it.
Try to search for keyword and phrase in the search engine databases. Note the number of search engine results. The more results, typically the more competitive the term. See the differences in number of search results for plural versions as opposed to singular versions of your keywords in each engine. Note the descriptions that the search engine results bring up and see whether any keyword phrases there that might apply to your website.
You have to work again with keywords and phrases list mach them into your web page. Incorporating these terms into your web pages should make good sense. At least most of them should realistically be part of the content of the page, not placed there only because you need them in the content. Have another person read your copy to see if it sounds reasonable to them.