What keyword did you pick?
You can use the google tools available on the adwords site (I think thats where they are)to see how much competition exists for a key word or phrase and how often it is searched for.
Hopefully, you haven't picked an incredibly popular search term thinking you can beat everyone else.
Also, there is a lot more to SEO than just picking a good key word. Have you done everything else you can to improve your rankings?
And, lastly, how long ago was this keyword selected? Some things just take time.
What is the keyword you selected? Is it the most relevant one? Is it relevant to your website topic? You have to also check for the keyword competition & average monhtly searches because a lot of things depends on that stats.
And, I think your website is not completed yet. Google always searches for relevancy of the keyword. So, in my opinion, first complete your website & then select the most relevant & appropriate keywords for your website from Google AdWords Tools Keywords Planner.
You should provide more details about your website and your keyword. initially focus on long tail keyword that will help you to rank on Search Engine, after a period of time you can focus on low level keywords too.