If you desire to make your website appear in top pages of search listing, you need to find out keywords that are most relevant to your website. Apart from your own imagination, there are three ways to figure out your keywords.
Learn From Your Competitors
This is the easiest method that would find many of the most important keywords. Go to a search engine and perform a search for websites that would cater same niche as yours or perform a search for the term that you think people would use to visit your site.
Open the website listed on top and let the home page download. After that click on "View" in browser bar and click "Source" or "Page Source" or right click on the page itself and click 'Source'. This would reveal the entire HTML used to build the web page, including all the keywords. Some of those keywords will be relevant to your site. Make a list of these keywords. Now visit the other site and find out more.
In blogging and web 2.0 sites, keywords are called tags. Blogs would display tag list in sidebar or in blog posts.
Pay Per Click Tools
Various pay per click sites would let you see how popular a keyword is. You can find competition for a keyword in terms of bidding price and number of searches made in Google AdWords and Yahoo. Here is link to Yahoo tool which is quite famous among the marketers.
For Google AdWords too, you would require to open an account with Adwords.
A number of websites supply keyword services for a fee. The best of these is WordTracker.com. This is quite expensive but quite rewarding.
GoogleFight.com let you compare two keywords and tells you which is more popular.
As you make up your list of keywords, you should also take into account key phrases that people might users. For example instead of "Home-Business, the user is more likely to search for 'how to do home business.' Key phrases are often overlooked, so if you do it well you increase your chances of top placement.
It is always a good idea to use more than one method to power your keyword search. Whichever of these methods you use you should be able to come up in the end with a pretty comprehensive list of keywords that you can use on your website.