Below are 5 things you can do to help you choose the best keywords to optimize your website for.  Always keep in mind that results are never instant when working on your SEO, so be patient and keep tract of changes you make so you can alter them in the future if you find they were more detrimental then helpful.

1.  Ask friends, family and clients what they would or have typed into a search engine to find a business like yours.
2.  Choose keywords that are relevant to the content of the individual webpage and try not to optimize different pages for the same exact keywords.  Spread out your coverage.
3.  Visit your successful competitors websites and see what they are optimized for by viewing their title tags and descriptions.  You can also use tools like SeoQuake to view more information about your competitors.
4.  Use a Keyword research tool like this Google Keyword Research Tool to find relevant keywords people are searching for that you have not thought about.  You can also use it to figure out what keywords Google is already seeing in your website.
5.  Take into consideration that there are many websites competing for the same words so if there are less popular but relevant keywords then optimize for those first.