Let's say I have some keywords in mind, but I am not sure how it will perform or exactly how to phrase them. I at least do the following to get a feel for what keywords and keyword phrases are the best. I borrowed from my own site for this - redacted of course.
Use the Google Search Text Box for ideas by typing in keyword combinations and exploring the suggestions that are returned within the list. These are hints from Google as to recent popular search history.
Use Google Trends to research keyword trends and popularity.
Google Trends displays a graph of interest over time. In this case, the interest has dropped off, but may still be enough for a site to compete.
Google Trends will show popular keyword search combinations. You can see we are using the keyword feline, but a popular keyword is cat.
I find this more interesting. If you click the "rising" button, you will see keyword trend information giving you a hint into the future. You can see the keywords symptoms, problems, and care. You will want to take note of these.
Search for each sets of keywords that make sense to you. Look at the links to see who competes well for these keywords. Then view the page source code and pay attention to the
description meta-tag, first
h1 tag, the first paragraph, and image tag alt text. Of course I am assuming you have some familiarity with HTML and browser features. Take notes on how the keywords are used. You may want to mimik this.
I also use tools such as Market Samurai. http://www.noblesamurai.com/ which is fairly good and sometimes SEO PowerSuite http://www.link-assistant.com/ which is better, but much more expensive and time consuming. I use Market Samurai the most because it is fast and simple to use though not always as complete sometimes for very hot trends.
I use all of these steps to help mull over keyword ideas. I usually get a pretty good picture doing this. Sometimes I just have to step back and think about it some more and may revisit any method along the way.
This should give you a good idea which way to go. Hope this helps. If you need more, let us know.